Human cloning
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Diagram of the ways to reprogram cells along with the development of humans

Human cloning is the creation of a genetically identical copy of a human. The term is generally used to refer to artificial human cloning, which is the reproduction of human cells and tissue. It does not refer to the natural conception and delivery of identical twins. The possibilities of human cloning have raised controversies. These ethical concerns have prompted several nations to pass laws regarding human cloning.

Two commonly discussed types of human cloning are therapeutic cloning and reproductive cloning.

Therapeutic cloning would involve cloning cells from a human for use in medicine and transplants. It is an active area of research, and is in medical practice over the world. Two common methods of therapeutic cloning that are being researched are somatic-cell nuclear transfer and (more recently) pluripotent stem cell induction.

Reproductive cloning would involve making an entire cloned human, instead of just specific cells or tissues.

History

[edit]

Although the possibility of cloning humans had been the subject of speculation for much of the 20th century, scientists and policymakers began to take the prospect seriously in 1969. J. B. S. Haldane was the first to introduce the idea of human cloning, for which he used the terms "clone" and "cloning",[1] which had been used in agriculture since the early 20th century. In his speech on "Biological Possibilities for the Human Species of the Next Ten Thousand Years" at the Ciba Foundation Symposium on Man and his Future in 1963, he said:[2]

It is extremely hopeful that some human cell lines can be grown on a medium of precisely known chemical composition. Perhaps the first step will be the production of a clone from a single fertilized egg, as in Brave New World... Assuming that cloning is possible, I expect that most clones would be made from people aged at least fifty, except for athletes and dancers, who would be cloned younger. They would be made from people who were held to have excelled in a socially acceptable accomplishment...

Nobel Prize-winning geneticist Joshua Lederberg advocated cloning and genetic engineering in an article in The American Naturalist in 1966 and again, the following year, in The Washington Post.[3] He sparked a debate with conservative bioethicist Leon Kass, who wrote at the time that "the programmed reproduction of man will, in fact, dehumanize him." Another Nobel Laureate, James D. Watson, publicized the potential and the perils of cloning in his Atlantic Monthly essay, "Moving Toward the Clonal Man", in 1971.[4]

With the cloning of a sheep known as Dolly in 1996 by somatic cell nuclear transfer (SCNT), the idea of human cloning became a hot debate topic.[5] Many nations outlawed it, while a few scientists promised to make a clone within the next few years. The first hybrid human clone was created in November 1998, by Advanced Cell Technology. It was created using SCNT; a nucleus was taken from a man's leg cell and inserted into a cow's egg from which the nucleus had been removed, and the hybrid cell was cultured and developed into an embryo. The embryo was destroyed after 12 days.[6]

In 2004 and 2005, Hwang Woo-suk, a professor at Seoul National University, published two separate articles in the journal Science claiming to have successfully harvested pluripotent, embryonic stem cells from a cloned human blastocyst using SCNT techniques. Hwang claimed to have created eleven different patient-specific stem cell lines. This would have been the first major breakthrough in human cloning.[7] However, in 2006 Science retracted both of his articles on account of clear evidence that much of his data from the experiments was fabricated.[8]

In January 2008, Dr. Andrew French and Samuel Wood of the biotechnology company Stemagen announced that they successfully created the first five mature human embryos using SCNT. In this case, each embryo was created by taking a nucleus from a skin cell (donated by Wood and a colleague) and inserting it into a human egg from which the nucleus had been removed. The embryos were developed only to the blastocyst stage, at which point they were studied in processes that destroyed them. Members of the lab said that their next set of experiments would aim to generate embryonic stem cell lines; these are the "holy grail" that would be useful for therapeutic or reproductive cloning.[9][10]

In 2011, scientists at the New York Stem Cell Foundation announced that they had succeeded in generating embryonic stem cell lines, but their process involved leaving the oocyte's nucleus in place, resulting in triploid cells, which would not be useful for cloning.[11][12][13]

In 2013, a group of scientists led by Shoukhrat Mitalipov published the first report of embryonic stem cells created using SCNT.[14] In this experiment, the researchers developed a protocol for using SCNT in human cells, which differs slightly from the one used in other organisms. Four embryonic stem cell lines from human fetal somatic cells were derived from those blastocysts. All four lines were derived using oocytes from the same donor, ensuring that all mitochondrial DNA inherited was identical.[11] A year later, a team led by Robert Lanza at Advanced Cell Technology reported that they had replicated Mitalipov's results and further demonstrated the effectiveness by cloning adult cells using SCNT.[5][15]

In 2018, the first successful cloning of primates using SCNT was reported with the birth of two live female clones, crab-eating macaques named Zhong Zhong and Hua Hua.[16][17]

Methods

[edit]

Somatic cell nuclear transfer (SCNT)

[edit]
Main article: Somatic cell nuclear transfer
Diagram of SCNT process

In somatic cell nuclear transfer ("SCNT"), the nucleus of a somatic cell is taken from a donor and transplanted into a host egg cell, which had its own genetic material removed previously, making it an enucleated egg. After the donor somatic cell genetic material is transferred into the host oocyte with a micropipette, the somatic cell genetic material is fused with the egg using an electric current. Once the two cells have fused, the new cell can be permitted to grow in a surrogate or artificially.[18] This is the process that was used to successfully clone Dolly the sheep (see § History).[5] The technique, now refined, has indicated that it was possible to replicate cells and reestablish pluripotency, or "the potential of an embryonic cell to grow into any one of the numerous different types of mature body cells that make up a complete organism".[19]

Induced pluripotent stem cells (iPSCs)

[edit]
Main article: Induced pluripotent stem cell
Overview of iPS cells

Creating induced pluripotent stem cells ("iPSCs") is a long and inefficient process. Pluripotency refers to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous tissue).[20] A specific set of genes, often called "reprogramming factors", are introduced into a specific adult cell type. These factors send signals in the mature cell that cause the cell to become a pluripotent stem cell. This process is highly studied and new techniques are being discovered frequently on how to improve this induction process.

Depending on the method used, reprogramming of adult cells into iPSCs for implantation could have severe limitations in humans. If a virus is used as a reprogramming factor for the cell, cancer-causing genes called oncogenes may be activated. These cells would appear as rapidly dividing cancer cells that do not respond to the body's natural cell signaling process. However, in 2008 scientists discovered a technique that could remove the presence of these oncogenes after pluripotency induction, thereby increasing the potential use of iPSC in humans.[21]

Comparing SCNT to reprogramming

[edit]

Both the processes of SCNT and iPSCs have benefits and deficiencies. Historically, reprogramming methods were better studied than SCNT derived embryonic stem cells (ESCs).[11] However, more recent studies have put more emphasis on developing new procedures for SCNT-ESCs. The major advantage of SCNT over iPSCs at this time is the speed with which cells can be produced. iPSCs derivation takes several months while SCNT would take a much shorter time, which could be important for medical applications. New studies are working to improve the process of iPSC in terms of both speed and efficiency with the discovery of new reprogramming factors in oocytes.[citation needed] Another advantage SCNT could have over iPSCs is its potential to treat mitochondrial disease, as it uses a donor oocyte.[11] No other advantages are known at this time in using stem cells derived from one method over stem cells derived from the other.[22]

Uses and actual potential

[edit]
Stem cell treatments

Work on cloning techniques has advanced understanding of developmental biology in humans. Observing human pluripotent stem cells grown in culture provides great insight into human embryo development, which otherwise cannot be seen. Scientists are now able to better define steps of early human development. Studying signal transduction along with genetic manipulation within the early human embryo has the potential to provide answers to many developmental diseases and defects. Many human-specific signaling pathways have been discovered by studying human embryonic stem cells. Studying developmental pathways in humans has given developmental biologists more evidence toward the hypothesis that developmental pathways are conserved throughout species.[23]

iPSCs and cells created by SCNT are useful for research into the causes of disease, and as model systems used in drug discovery.[24][25]

Cells produced with SCNT, or iPSCs could eventually be used in stem cell therapy,[26] or to create organs to be used in transplantation, known as regenerative medicine. Stem cell therapy is the use of stem cells to treat or prevent a disease or condition. Bone marrow transplantation is a widely used form of stem cell therapy.[27] No other forms of stem cell therapy are in clinical use at this time. Research is underway to potentially use stem cell therapy to treat heart disease, diabetes, and spinal cord injuries.[28][29] Regenerative medicine is not in clinical practice, but is heavily researched for its potential uses. This type of medicine would allow for autologous transplantation, thus removing the risk of organ transplant rejection by the recipient.[30] For instance, a person with liver disease could potentially have a new liver grown using their same genetic material and transplanted to remove the damaged liver.[31] In current research, human pluripotent stem cells have been promised as a reliable source for generating human neurons, showing the potential for regenerative medicine in brain and neural injuries.[32]

Ethical implications

[edit]
Main article: Ethics of cloning

In bioethics, the ethics of cloning refers to a variety of ethical positions regarding the practice and possibilities of cloning, especially human cloning. While many of these views are religious in origin, for instance relating to Christian views of procreation and personhood,[33] the questions raised by cloning engage secular perspectives as well, particularly the concept of identity.[34]

Advocates support development of therapeutic cloning in order to generate tissues and whole organs to treat patients who otherwise cannot obtain transplants,[35] to avoid the need for immunosuppressive drugs,[36] and to stave off the effects of aging.[37] Advocates for reproductive cloning believe that parents who cannot otherwise procreate should have access to the technology.[38]

Opposition to therapeutic cloning mainly centers around the status of embryonic stem cells, which has connections with the abortion debate.[39] The moral argument put forward is based on the notion that embryos deserve protection from the moment of their conception because it is at this precise moment that a new human entity emerges, already a unique individual.[40] Since it is deemed unacceptable to sacrifice human lives for any purpose, the argument asserts that the destruction of embryos for research purposes is no longer justifiable.[41]

Some opponents of reproductive cloning have concerns that technology is not yet developed enough to be safe – for example, the position of the American Association for the Advancement of Science as of 2014,[42] while others emphasize that reproductive cloning could be prone to abuse (leading to the generation of humans whose organs and tissues would be harvested),[43][44] and have concerns about how cloned individuals could integrate with families and with society at large.[45][46]

Members of religious groups are divided. Some Christian theologians perceive the technology as usurping God's role in creation and, to the extent embryos are used, destroying a human life;[33] others see no inconsistency between Christian tenets and cloning's positive and potentially life-saving benefits.[47][48]

[edit]
Human cloning laws
  Illegal
  Some forms legal
  Legal
  No data
Human therapeutic cloning laws by U.S. state
  Legal
  Illegal
  No data, not specified or unclear
[edit]
Jurisdiction Reproductive cloning Therapeutic cloning Notes
Argentina Illegal[49][50] Illegal[50] Human cloning is banned by the Presidential Decree 200/97 of 7 March 1997.[49]
Australia Illegal[51][50] Legal[52] Australia has prohibited human cloning,[53] though as of December 2006, a bill legalizing therapeutic cloning and the creation of human embryos for stem cell research passed the House of Representatives. Within certain regulatory limits, and subject to the effect of state legislation, therapeutic cloning is now legal in some parts of Australia.[51]
Austria Illegal[50] Illegal[50]
Belgium Illegal[51] Legal[54][50]
Brazil Illegal[50] Illegal[50]
Canada Illegal[55][50] Illegal[55][50] Canadian law prohibits the following: cloning humans, cloning stem cells, growing human embryos for research purposes, and buying or selling of embryos, sperm, eggs or other human reproductive material.[55] It also bans making changes to human DNA that would pass from one generation to the next,[56] including use of animal DNA in humans. Surrogate mothers are legally allowed, as is donation of sperm or eggs for reproductive purposes. Human embryos and stem cells are also permitted to be donated for research.[57]

There have been consistent calls in Canada to ban human reproductive cloning since the 1993 Report of the Royal Commission on New Reproductive Technologies. Polls have indicated that an overwhelming majority of Canadians oppose human reproductive cloning, though the regulation of human cloning continues to be a significant national and international policy issue. The notion of "human dignity" is commonly used to justify cloning laws. The basis for this justification is that reproductive human cloning necessarily infringes notions of human dignity.[58][59][60][61]

Chile Illegal[50] Illegal[50]
China Illegal[62][50] Legal[54][50] The government "does not approve, does not allow, does not support, does not accept" any reproductive human cloning experiments, but does not oppose therapeutic cloning.[62]

In the Eleventh Amendment to the Criminal Law, which came into effect on March 1, 2021, an additional provision was added to Article 336, which stipulates that "implanting gene-edited or cloned human embryos into human or animal bodies, or implanting gene-edited, cloned Implantation of cloned animal embryos into human bodies, if the circumstances are serious, shall be sentenced to fixed-term imprisonment of not more than three years or criminal detention and a fine; if the circumstances are especially serious, the sentence shall be fixed-term imprisonment of not less than three years but not more than seven years and a fine."[63]

Colombia Illegal[64] Legal[50] Human cloning is prohibited in Article 133 of the Colombian Penal Code.[64]
Costa Rica Illegal[50] Illegal[50]
Council of Europe Illegal[65] Not specified[65] The European Convention on Human Rights and Biomedicine, a.k.a. the Oviedo Convention, prohibits human cloning in one of its additional protocols;[65] this protocol has been ratified by the following states:[65][66]

Albania, Andorra, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Greece, Hungary, Iceland, Latvia, Liechtenstein, Lithuania, Moldova, Montenegro, North Macedonia, Norway, Portugal, Romania, San Marino, Serbia, Slovakia, Slovenia, Spain, Switzerland, Turkey

Czech Republic Illegal[50] Illegal[50]
Denmark Illegal[50] Illegal[50]
Ecuador Illegal[50] Illegal[50]
Egypt Illegal[50] Illegal[50]
Estonia Illegal[50] Illegal[50]
European Union Illegal[67] Not specified[67] The Charter of Fundamental Rights of the European Union explicitly prohibits reproductive human cloning. The charter is legally binding for the institutions of the European Union under the Treaty of Lisbon and for some member countries of the Union implementing EU regulations.[a][68][69]
Finland Illegal[65][66] Legal[50]
France Illegal[70][50] Illegal[70][50] The Code Civil in its article 16-4 prohibits all forms of cloning. All forms of cloning including therapeutic cloning has been specifically prohibited by 6 August 2004 bioethics law[70]
Georgia Illegal[50] Illegal[50]
Germany Illegal[50][71][72] Illegal[50][71][72]
Greece Illegal[50] Illegal[50]
Hungary Illegal[73] Not specified[50] The Constitution of Hungary (Section Freedom and Responsibility, Article 3 (3)) prohibits human cloning.[73]
Iceland Illegal[50] Illegal[50]
India Illegal[74] Legal[54] India does not have specific laws regarding cloning but has guidelines prohibiting whole human cloning or reproductive cloning. India allows therapeutic cloning and the use of embryonic stem cells for research purposes. There are legal implications in this case.[75][74]

India has already succeeded in mammalian cloning.[76]

Iran No data Not specified[50]
Ireland Illegal[50] Illegal[50]
Israel Illegal[77] Legal[50][54]
Italy Illegal[50] Illegal[50]
Japan Illegal[78] Legal[50][54]
Latvia Illegal[50] Illegal[50]
Lithuania Illegal[50] Illegal[50]
Morocco Illegal[79] Illegal[79]

In Morocco, all research on human embryos or fetuses is forbidden, as is the conception of human embryos or fetuses for research or experimental purposes, in accordance with article 7 of Dahir no. 1–19–50.[79]

Netherlands Illegal[50] Illegal[50]
New Zealand No data Legal[50]
Norway Illegal[50] Illegal[50]
Pakistan Illegal[80] Legal within the limits of Islam[80] Pakistan's Council of Islamic Ideology has declared human cloning as an un-Islamic act. According to Pakistan's Council of Islamic Ideology, research and thinking are not banned in Islam; new innovations are allowed, but within the limits of the religion.[80]
Panama Illegal[50] Illegal[50]
Peru Illegal[50] Illegal[50]
Poland Illegal[81][50] Illegal[50] Human cloning forbidden by article 87 of Act of 25 June 2015.[81]
Portugal Illegal[50] Illegal[50]
Republic of China Illegal[50] Illegal[50]
Russia Illegal[50][82] Illegal[50] The Federal Assembly of Russia introduced the Federal Law N 54-FZ "On the temporary ban on human cloning" on 19 April 2002. On 20 May 2002, President Vladimir Putin signed this moratorium on the implementation of human cloning. On 29 March 2010, The Federal Assembly introduced second revision of this law without time limit.[82]
Serbia Illegal[83] No data Human cloning is explicitly prohibited in Article 24, "Right to Life" of the 2006 Constitution of Serbia.[83]
Singapore Illegal[84] Legal[54][50] Section 5 of the Human Cloning and Other Prohibited Practices Act 2004 prohibits the placing of a human embryo clone in the body of a human or animal.[84]
Slovakia Illegal[50] Illegal[50]
Slovenia Illegal[50] Illegal[50]
South Africa Illegal[85][86][50] Illegal[85][86][50] In terms of section 39A of the Human Tissue Act 65 of 1983,[85] genetic manipulation of gametes or zygotes outside the human body is absolutely prohibited. A zygote is the cell resulting from the fusion of two gametes; thus the fertilised ovum. Section 39A thus prohibits human cloning.[86]
South Korea Illegal[87] Legal[54][50]
Spain Illegal[50] Illegal[50]
Sweden Illegal[88] Legal[50][54]
 Switzerland Illegal[51][50] Illegal[51][50]
Thailand No data Legal[50]
Trinidad and Tobago Illegal[50] Illegal[50]
Tunisia Illegal[50] Illegal[50]
Turkey No data Legal[50]
Ukraine No data Not specified[50]
United Kingdom Illegal[51] Legal[54][50] On 14 January 2001, the British government passed The Human Fertilisation and Embryology (Research Purposes) Regulations 2001[89] to amend the Human Fertilisation and Embryology Act 1990 by extending allowable reasons for embryo research to permit research around stem cells and cell nuclear replacement, thus allowing therapeutic cloning. However, on 15 November 2001, a pro-life group won a High Court legal challenge, which struck down the regulation and effectively left all forms of cloning unregulated in the UK. Their hope was that Parliament would fill this gap by passing prohibitive legislation.[90][91] Parliament was quick to pass the Human Reproductive Cloning Act 2001 which explicitly prohibited reproductive cloning. The remaining gap with regard to therapeutic cloning was closed when the appeals courts reversed the previous decision of the High Court.[92]

The first license was granted on 11 August 2004, to researchers at the University of Newcastle to allow them to investigate treatments for diabetes, Parkinson's disease and Alzheimer's disease.[93] The Human Fertilisation and Embryology Act 2008, a major review of fertility legislation, repealed the 2001 Cloning Act by making amendments of similar effect to the 1990 Act. The 2008 Act also allows experiments on hybrid human-animal embryos.[94]

United Nations Illegal[95] (soft law, i.e. not binding on member states) Not specified[95] On 13 December 2001, the United Nations General Assembly began elaborating an international convention against the reproductive cloning of humans. A broad coalition of states, including Spain, Italy, the Philippines, the United States, Costa Rica, and the Holy See sought to extend the debate to ban all forms of human cloning, noting that, in their view, therapeutic human cloning violates human dignity. Costa Rica proposed the adoption of an international convention to ban all forms of human cloning. Unable to reach a consensus on a binding convention, in March 2005 a non-binding United Nations Declaration on Human Cloning, calling for the ban of all forms of human cloning contrary to human dignity, was adopted.[96][97]
United States Not legislated at the
federal level, depends on
state legislation[98][51] 		Not legislated at the
federal level, depends on
state legislation[51][98][50] 		As of 2024, there are no federal laws in the United States which ban cloning completely.[51][98]

In 1998, 2001, 2004, 2005, 2007 and 2009, the United States Congress voted whether to ban all human cloning, both reproductive and therapeutic (Stem Cell Research Enhancement Act).[99] Divisions in the Senate, or an eventual veto from the sitting President (George W. Bush in 2005 and 2007), over therapeutic cloning prevented either competing proposal (a ban on both forms or on reproductive cloning only) from being passed into law. On 10 March 2010, a bill (HR 4808) was introduced with a section banning federal funding for human cloning.[100] Such a law, if passed, would not have prevented research from occurring in private institutions (such as universities) that have both private and federal funding. However, the 2010 law was not passed.

Ten states, California, Connecticut, Illinois, Iowa, Maryland, Massachusetts, Missouri, Montana, New Jersey and Rhode Island, have "clone and kill" laws that prevent cloned embryo implantation for childbirth, but allow embryos to be destroyed.[101]

The Patients First Act of 2017 (HR 2918, 115th Congress) aims to promote stem cell research, using cells that are "ethically obtained", that could contribute to a better understanding of diseases and therapies, as well as promote the "derivation of pluripotent stem cell lines without the creation of human embryos".[102]

Uruguay No data Not specified[50]
Vietnam Illegal[50] Illegal[50]
[edit]
State Legal status Notes
Reproductive cloning Therapeutic cloning
 Alabama Not legislated[103] Not legislated[103]
 Alaska Not legislated[103] Not legislated[103]
 Arizona Illegal[103] Illegal[103] Prohibition on the use of public funds for human cloning.[104]
 Arkansas Illegal[103][104][105][106] Illegal[103][105][106] Criminal and civil penalties.[105][106]
 California Illegal[103][104][105] Legal[103][101] Civil penalties.[105]
 Colorado Not legislated[103] Not legislated[103]
 Connecticut Illegal[103][104] Legal[103][101]
 Delaware Not legislated[103] Not legislated[103]
 Florida Unclear[103][104] Not legislated[103]
 Georgia Unclear[103][104] Not legislated[103]
 Hawaii Not legislated[103] Not legislated[103]
 Idaho Not prohibited[103] Not prohibited[103]
 Illinois Illegal[103] Legal[103][101]
 Indiana Illegal (indirectly)[103][104] Illegal (indirectly)[103] Prohibition on the use of public funds for human cloning.[103]
 Iowa Illegal[103][104][105][107] Unclear[103][101][105][107] Criminal and civil penalties.[105][107]
 Kansas Not legislated[103] Not legislated[103]
 Kentucky[A] Not legislated[103] Not legislated[103]
 Louisiana Law prohibiting expired[103][105][108] Law allowing expired[103] Criminal and civil penalties.[105][108] Prohibition on the use of public funds for human cloning.[103]
 Maine Illegal (indirectly)[103] Illegal (indirectly)[103]
 Maryland Illegal[103][104] Legal[103][101] Prohibition on the use of public funds for human cloning.[103][104]
 Massachusetts[A] Illegal[103][104] Legal[103][101]
 Michigan Illegal[103][104][105][109] Illegal[103][105][109] Criminal and civil penalties.[105][109]
 Minnesota Not legislated[103] Illegal (indirectly)[103]
 Mississippi Not legislated[103] Not legislated[103]
 Missouri Illegal[103] Legal[103][101] Prohibition on the use of public funds for human cloning.[104]
 Montana Illegal[103] Legal[103][101]
 Nebraska Not legislated[103] Not legislated[103] Prohibition on the use of public funds for human cloning.[103]
 Nevada Not legislated[103] Not legislated[103]
 New Hampshire Not legislated[103] Not legislated[103]
 New Jersey Illegal[103][104] Legal[103][101]
 New Mexico Not legislated[103] Not legislated[103]
 New York Not legislated[103] Not legislated[103]
 North Carolina Not legislated[103] Not legislated[103]
 North Dakota Illegal[103][105][104] Illegal[103][105] Criminal and civil penalties.[105]
 Ohio Not legislated[103] Not legislated[103]
 Oklahoma Illegal[103] Illegal[103]
 Oregon Not legislated[103] Not legislated[103]
 Pennsylvania[A] Not legislated[103] Not legislated[103]
 Rhode Island Illegal[103][105][104][110] Legal[103][101] Criminal and civil penalties.[105][110]
 South Carolina Not legislated[103] Not legislated[103]
 South Dakota Illegal[103][104] Illegal[103]
 Tennessee Not legislated[103] Not legislated[103]
 Texas Not legislated[103] Not legislated[103]
 Utah Not legislated[103] Not legislated[103]
 Vermont Not legislated[103] Not legislated[103]
 Virginia[A] Illegal[104][105][111] Unclear[103][105] Civil penalties.[105][111]
 Washington Not legislated[103] Not legislated[103]
 West Virginia Not legislated[103] Not legislated[103]
 Wisconsin Not legislated[103] Not legislated[103]
 Wyoming Not legislated[103] Not legislated[103]
[edit]
Further information: Cloning § In popular culture

Science fiction has used cloning, most commonly and specifically human cloning, due to the fact that it brings up controversial questions of identity.[112][113] Humorous fiction, such as Multiplicity (1996)[114] and the Maxwell Smart feature The Nude Bomb (1980), have featured human cloning.[115] A recurring sub-theme of cloning fiction is the use of clones as a supply of organs for transplantation. Robin Cook's 1997 novel Chromosome 6, Michael Bay's The Island, and Nancy Farmer's 2002 novel House of the Scorpion[116] are examples of this; Chromosome 6 also features genetic manipulation and xenotransplantation.[117] The Star Wars saga makes use of millions of human clones to form the Grand Army of the Republic that participated in the Clone Wars. The series Orphan Black follows human clones' stories and experiences as they deal with issues and react to being the property of a chain of scientific institutions.[118] In the 2019 horror film Us, the entirety of the United States' population is secretly cloned. Years later, these clones (known as The Tethered) reveal themselves to the world by successfully pulling off a mass genocide of their counterparts.[119][120]

In the 2005 novel Never Let Me Go, Kazuo Ishiguro crafts a subtle exploration into the ethical complications of cloning humans for medical advancement and longevity.

See also

[edit]

Notes and references

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Human cloning

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Human cloning involves the asexual replication of human genetic material to produce identical copies of cells, embryos, or potentially full individuals, primarily through somatic cell nuclear transfer (SCNT), where the nucleus of a somatic cell is inserted into an enucleated oocyte.[1][2] This process distinguishes between reproductive cloning, intended to yield a viable human organism genetically identical to the donor, and therapeutic cloning, aimed at generating embryonic stem cells for medical applications without gestation.[3] Despite successes in animal models, such as the 1996 cloning of Dolly the sheep, no verified instance of human reproductive cloning has been achieved, with reported attempts marred by scientific fraud or lack of substantiation.[4] Therapeutic cloning has advanced further, enabling the derivation of patient-specific stem cells from cloned human embryos as early as 2013, though efficiency remains low and ethical concerns persist regarding embryo viability and destruction.[5][6] The technique's development stems from foundational work in nuclear transfer, but human applications face profound biological hurdles, including high rates of developmental abnormalities observed in cloned animals, epigenetic reprogramming failures, and premature aging syndromes.[7] Controversies encompass not only these technical inefficiencies but also profound ethical debates over human dignity, the commodification of life, and risks of psychological harm to clones, such as identity crises or societal stigmatization.[8] Legally, reproductive human cloning is prohibited in approximately 46 countries, with many others imposing moratoriums or restrictions on therapeutic forms, reflecting global consensus on its perils amid divergent views on embryo status.[9][10] While proponents argue potential benefits for infertility treatment or organ regeneration, empirical data underscore unresolved safety issues, underscoring cloning's status as a frontier technology constrained by both science and principle.[11]

Definitions and Scope

Core Definitions

Human cloning is the artificial creation of a genetically identical copy of a human organism or its cells, typically through techniques that replicate the nuclear DNA from a donor somatic cell.[1] This process differs from natural identical twinning, which occurs spontaneously during embryonic development without human intervention.[2] The most discussed method for human cloning is somatic cell nuclear transfer (SCNT), in which the nucleus of a donor somatic cell is transferred into an enucleated oocyte (egg cell), followed by chemical or electrical stimulation to initiate division.[12][13] Reproductive cloning refers to the use of such techniques to produce a viable, live-born human genetically identical to the donor, with the cloned embryo implanted into a uterus for gestation.[1][2] In contrast, therapeutic cloning—also known as somatic cell nuclear transfer for research—generates cloned embryos solely to derive embryonic stem cells for medical applications, such as tissue repair or disease modeling, without intent to create a born individual.[1][11] A third category, gene or molecular cloning, involves replicating specific DNA segments or genes in vitro for research purposes but does not produce whole organisms or cells and is not typically encompassed under "human cloning" in ethical or policy discussions.[14][1] These definitions emphasize genetic identity at the nuclear level, though mitochondrial DNA from the oocyte donor may introduce minor variations; full genomic identity would require cloning the donor's oocyte as well.[12] No verified instances of successful human reproductive cloning exist as of 2025, distinguishing it empirically from animal cloning successes like Dolly the sheep in 1996.[1]

Types of Human Cloning

Human cloning is classified into reproductive cloning and therapeutic cloning, with the latter sometimes encompassing research applications; a third category, gene cloning, involves replicating human DNA segments but does not produce organisms or embryos.[1] Reproductive cloning aims to generate a complete, genetically identical human individual by transferring a somatic cell nucleus into an enucleated egg, developing the resulting embryo to the blastocyst stage, and implanting it into a uterus for gestation.[2] This process mirrors the technique used to clone Dolly the sheep in 1996, but applied to humans, with the intent of producing a viable offspring.[1] No verified instances of successful human reproductive cloning have been documented as of 2023, due to technical inefficiencies, high failure rates observed in animal models, and ethical prohibitions in most jurisdictions.[12] Therapeutic cloning, also termed somatic cell nuclear transfer for research or embryo cloning, creates cloned embryos specifically to derive patient-matched embryonic stem cells for regenerative medicine or disease modeling, without plans for implantation or birth.[11] The cloned embryo is cultured to the blastocyst stage, from which the inner cell mass is extracted to produce pluripotent stem cells capable of differentiating into various tissue types, potentially avoiding immune rejection in therapies.[1] This method has been pursued to treat conditions like Parkinson's disease or spinal cord injuries, though human applications remain experimental and limited by low success rates in nuclear reprogramming, typically below 5% in mammalian models.[15] Advances in induced pluripotent stem cells have partially supplanted therapeutic cloning needs, offering non-embryonic alternatives for cell generation.[3] Gene cloning, distinct from organismal cloning, replicates specific human genes or DNA fragments using recombinant DNA technology in host organisms like bacteria or yeast, primarily for studying gene function, producing proteins, or gene therapy vectors.[1] While integral to molecular biology and human genomics research, such as the Human Genome Project completed in 2003, it does not involve creating human embryos or individuals and thus falls outside the scope of debates on human reproductive or therapeutic cloning.[1]

Historical Development

Early Theoretical Foundations

The theoretical foundations of human cloning trace back to late 19th- and early 20th-century embryological studies on cellular totipotency, which demonstrated that individual cells from early embryos could develop into complete organisms. In 1885, Hans Driesch separated the blastomeres of two-celled sea urchin embryos, showing that each cell retained the full genetic potential to form a viable larva, thus establishing artificial embryo twinning as a form of cloning.[4] This work highlighted the regulative capacity of early embryonic cells, a principle later extended to vertebrates. In 1902, Hans Spemann replicated twinning in salamander embryos by constricting fertilized eggs with a fine hair loop, producing identical twins from cells at the two- to four-cell stage and confirming that early blastomeres in higher animals possess totipotent nuclei capable of directing full development.[4] Spemann's research advanced further with experiments on embryonic induction, culminating in the 1924 "organizer" concept—where dorsal lip tissue from a salamander gastrula induced a second embryo when transplanted—earning him the 1935 Nobel Prize in Physiology or Medicine.[16] In 1928, Spemann performed rudimentary nuclear transfers using salamander embryos, relocating nuclei from early cleavage stages into enucleated eggs and observing development guided by the transplanted nucleus, which underscored the nucleus's dominant role in heredity over cytoplasmic factors.[4] These findings challenged August Weismann's 1892 germ plasm theory, which posited irreversible differentiation and separation of germinal from somatic lines, by suggesting nuclei might retain developmental plasticity. The pivotal theoretical proposal for cloning differentiated cells came in Spemann's 1938 book Embryonic Development and Induction, where he outlined a "fantastical experiment": transplant the nucleus of a differentiated embryonic cell into an enucleated unfertilized egg of the same species to determine if it could orchestrate normal ontogeny.[16] Spemann viewed this as technically unfeasible at the time due to the inability to micromanipulate mammalian or human cells precisely, but it formalized the hypothesis that somatic nuclei could be reprogrammed by egg cytoplasm to regain totipotency. This concept directly anticipated somatic cell nuclear transfer (SCNT), the technique central to later animal cloning and proposed for human reproductive cloning, implying that human development could theoretically be replicated from a differentiated donor nucleus if ethical and technical barriers were overcome.[16]

Key Experimental Milestones

The first reported attempt to clone human embryos using somatic cell nuclear transfer (SCNT) occurred in November 2001, when researchers at Advanced Cell Technology announced the creation of embryos from human leg cells that developed to the six-cell stage before arresting; however, independent verification was limited, and the embryos were not viable for further development.[17] In 2004, South Korean scientist Hwang Woo-suk claimed to have derived patient-specific human embryonic stem cell lines via SCNT, reporting the creation of 11 lines from 242 oocytes using cumulus cells; these results, published in Science, were hailed as a breakthrough in therapeutic cloning but were later retracted in 2006 after evidence emerged of data fabrication, ethical violations in egg procurement, and no actual stem cell derivation.[18] A verified advancement came in 2008, when a team led by Andrew French and Samuel Wood at Stemagen Corporation reported the creation of five human blastocyst-stage embryos via SCNT using skin cells from female donors and oocytes from the same donors, though no embryonic stem cell lines were derived due to technical limitations in culturing. The first confirmed derivation of human embryonic stem cells through SCNT was achieved in May 2013 by Shoukhrat Mitalipov's team at Oregon Health & Science University, who transferred nuclei from fetal somatic cells into enucleated human oocytes, yielding viable blastocysts from which two pluripotent stem cell lines were established; this demonstrated efficient reprogramming in human cells, overcoming prior inefficiencies seen in animal models.00384-9)[19] Subsequent refinements included a 2014 Japanese study by Masato Nakagawa and team, who successfully generated induced pluripotent stem cells as a comparison but also advanced SCNT protocols to derive stem cells from adult human fibroblasts, confirming the 2013 findings with higher efficiency using caffeine to prevent premature activation. No verified reproductive human cloning—intended to produce a live birth—has occurred, with all claims, such as the 2002 announcement by Clonaid of a cloned infant named "Eve," lacking empirical evidence or independent confirmation.[1]

Modern Attempts and Claims

In December 2002, Clonaid, a company affiliated with the Raelian religious group, announced the birth of a baby girl named Eve, claimed to be the world's first human clone produced via somatic cell nuclear transfer from her 31-year-old American mother's DNA.[20] [21] Clonaid's CEO, Brigitte Boisselier, stated the cloning occurred outside the United States and that Eve was healthy, but provided no DNA evidence, medical records, or independent verification, citing privacy concerns.[22] [23] By 2004, Clonaid escalated claims to having produced 14 human clones, yet refused offers for third-party genetic testing, leading to widespread dismissal by scientists as unsubstantiated publicity.[24] Concurrent efforts involved Italian fertility specialist Severino Antinori, who in April 2002 claimed one of his patients was two months pregnant with a cloned fetus, intending to produce the first cloned baby by late 2002 or early 2003 to aid infertile couples.[25] [26] Antinori, previously known for pioneering preimplantation genetic diagnosis, partnered with Cypriot-American physician Panayiotis Zavos in 2001 to pursue human reproductive cloning using donated eggs and somatic cells from infertile individuals.[27] These announcements drew immediate condemnation from scientific bodies, including the European Society of Human Reproduction and Embryology, for ethical violations and safety risks, with no subsequent birth confirmations or peer-reviewed data emerging.[28] Zavos continued independent claims into the 2000s; in February 2004, he reported implanting a cloned embryo into a woman, which he later admitted failed to result in pregnancy.[29] By April 2009, Zavos asserted he had cloned 14 human embryos, implanting 11 into four women's uteri, predicting clones within months, but offered no verifiable proof beyond self-reported procedures conducted in undisclosed locations to evade bans.[30] [31] Leading reproductive medicine experts, such as those from the American Society for Reproductive Medicine, rejected these as reckless and unscientific, noting high failure rates in animal cloning (e.g., over 90% embryonic loss) render human viability improbable without transparency.[32] No credible, verified instances of human reproductive cloning have occurred as of 2025, with major institutions like the National Human Genome Research Institute affirming such claims remain fictional amid persistent technical barriers like incomplete epigenetic reprogramming and elevated abnormality risks observed in mammalian clones.[1] Efforts by entities like Advanced Cell Technology in the early 2000s focused on therapeutic cloning of early-stage embryos (up to six cells) rather than viable births, yielding no reproductive outcomes.[33] Post-2010, public claims have dwindled, supplanted by advances in induced pluripotent stem cells and gene editing, though rogue assertions persist without empirical substantiation.[24]

Scientific Methods and Techniques

Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) is a cloning technique that involves transferring the nucleus of a somatic (non-reproductive) cell into an enucleated oocyte, or egg cell, to reprogram the donor nucleus and initiate embryonic development.[34] The process begins with isolating a mature oocyte from a donor female and removing its nucleus using micromanipulation tools, creating a cytoplast devoid of genetic material.[34] A somatic cell nucleus, typically from skin fibroblasts or other easily accessible tissues, is then inserted into the enucleated oocyte via electrofusion or microinjection.[34] Chemical or electrical activation follows to mimic fertilization, prompting the reconstructed embryo to divide and potentially form a blastocyst.[34] In therapeutic cloning applications, the SCNT-derived blastocyst is used to generate patient-matched embryonic stem cells, which can differentiate into various cell types without triggering immune rejection.[35] The first derivation of human embryonic stem cells via SCNT occurred in 2013, when researchers used fetal somatic cells and oocytes from eight donors, achieving a 10% blastocyst formation rate from 104 reconstructed embryos.[35] For reproductive cloning, the embryo is implanted into a surrogate uterus to develop into a full organism genetically identical to the somatic cell donor, though no verified human successes exist.[36] SCNT efficiency remains low due to incomplete epigenetic reprogramming, where the somatic nucleus fails to fully reset to an embryonic state, leading to aberrant gene expression and developmental arrest.[37] In animal models, live birth rates range from 1-5% of transferred embryos, with higher rates up to 20% in optimized bovine protocols using specific donor cell types.[36] Common abnormalities include large offspring syndrome, placental defects, and premature aging, attributed to persistent DNA methylation errors.[34] Human applications face additional hurdles, such as limited oocyte availability and ethical restrictions, restricting progress to therapeutic contexts.[38] Advances in reprogramming factors, like histone deacetylase inhibitors, have incrementally improved blastocyst yields in primates, but full-term viability in humans remains unachieved.[37]

Stem Cell Reprogramming and Alternatives

Induced pluripotent stem cells (iPSCs) represent a primary alternative to somatic cell nuclear transfer (SCNT) for generating pluripotent cells suitable for therapeutic cloning applications. In 2006, Shinya Yamanaka and colleagues demonstrated that introducing four transcription factors—Oct4, Sox2, Klf4, and c-Myc—into mouse embryonic or adult fibroblasts could reprogram them into a pluripotent state resembling embryonic stem cells.[39] This breakthrough was extended to human cells in 2007, when the same factors successfully reprogrammed adult human dermal fibroblasts into iPSCs capable of forming teratomas and contributing to chimeric mice.[40] Unlike SCNT, which requires unfertilized oocytes and results in cloned embryos, iPSC generation uses readily available somatic cells from the patient, avoiding ethical concerns over embryo destruction and oocyte donation scarcity.[41] iPSCs enable the production of autologous pluripotent cells for disease modeling, drug screening, and regenerative therapies without nuclear transfer. These cells can differentiate into virtually any cell type, supporting applications like personalized medicine where patient-derived iPSCs are used to study genetic diseases or test treatments.[42] Advances since 2007 include non-integrating reprogramming methods, such as Sendai virus vectors or mRNA delivery, which reduce risks of genomic insertion mutations associated with early retroviral approaches.[42] By 2024, chemical reprogramming protocols—using small molecules to replace transcription factors—have improved efficiency and safety, though yields remain lower than viral methods at around 0.01-1% for human cells.[42] Despite these developments, iPSC technology faces biological limitations compared to SCNT-derived cells. Reprogrammed iPSCs often retain epigenetic memory from their somatic origin, leading to biased differentiation toward the donor cell type and incomplete maturation of derivatives, which hampers their use in modeling adult-onset diseases.[42] Tumorigenicity poses a major risk, as undifferentiated iPSCs or incompletely reprogrammed cells can form teratomas upon transplantation, with rates exceeding 20% in early mouse studies; human trials require rigorous purification to mitigate this.[43] Genetic aberrations, including copy number variations, accumulate during passaging, with mutation rates up to 10-20 per exome in long-term cultures, necessitating clonal selection and genomic screening.[44] Direct cellular reprogramming, or transdifferentiation, offers another alternative by converting somatic cells into specific lineages without passing through pluripotency, potentially sidestepping teratoma risks. For instance, fibroblasts have been directly reprogrammed into neurons or cardiomyocytes using lineage-specific transcription factors like Ascl1, Brn2, and Myt1l, achieving up to 20% efficiency in human cells by 2023.[45] This method preserves cell identity better than iPSCs but yields lineage-restricted cells unsuitable for broad therapeutic cloning goals. Hybrid approaches, such as combining iPSC technology with SCNT (iPSC-NT), have been explored to enhance reprogramming fidelity, producing cells with fewer epigenetic errors than either alone, though clinical translation remains preclinical as of 2025.[46] Overall, while iPSCs have largely supplanted SCNT in research due to accessibility, unresolved challenges in safety and fidelity limit their equivalence for human cloning applications.00445-4)

Technical Comparisons and Limitations

Somatic cell nuclear transfer (SCNT) and induced pluripotent stem cell (iPSC) reprogramming represent primary techniques for generating patient-specific pluripotent cells, with SCNT involving the transfer of a somatic nucleus into an enucleated oocyte to initiate embryonic development, while iPSC generation reprograms somatic cells directly via transcription factors such as Oct4, Sox2, Klf4, and c-Myc without requiring oocytes.[37] SCNT leverages the oocyte's natural reprogramming machinery, potentially yielding more complete epigenetic erasure compared to iPSC methods, which rely on exogenous factors and exhibit persistent somatic epigenetic memory or heterogeneity.[37] However, iPSC reprogramming achieves higher initial efficiencies—often 0.01-1% for colony formation versus SCNT's embryo development rates below 5% in mammals—making it more scalable for therapeutic applications, though iPSCs carry risks of insertional mutagenesis from viral vectors or incomplete silencing of reprogramming factors.[42][47] In human contexts, SCNT has produced embryonic stem cell lines from adult fibroblasts, as demonstrated in 2013 with fetal cells and 2014 with adult dermal cells, confirming pluripotency and low immunogenicity for autologous use, but these efforts required hundreds of oocytes per line due to arrest at early cleavage stages.[48] iPSC methods, conversely, bypass oocyte dependency, enabling rapid derivation from accessible tissues like blood or skin, with human iPSC lines routinely generated since 2007 at efficiencies improved to over 1% via non-integrating vectors.[49] Yet, comparative genomic analyses reveal iPSCs accumulate more mutations and epigenetic aberrations than SCNT-derived cells, potentially limiting their fidelity for modeling or transplantation.[50] Key limitations of SCNT include profoundly low efficiency—typically 1-5% live birth rates in animal models like mice (around 2%) and pigs (1%), extrapolated to humans where no viable reproductive clones exist—and persistent epigenetic defects such as aberrant DNA methylation and histone modifications (e.g., H3K9me3 retention), causing developmental arrest, placental abnormalities, and post-natal syndromes like large offspring syndrome.[51][52][53] These stem from incomplete nuclear reprogramming, with somatic chromatin barriers resisting oocyte-mediated erasure, necessitating chemical inhibitors or donor age matching to modestly boost yields.[54] iPSC limitations encompass genetic instability from reprogramming-induced mutations (up to 10-20 per genome) and reduced differentiation potential due to bivalent domains or residual transgene expression, though these are mitigated in SCNT-iPSC hybrids that combine efficiencies.[42][46] Overall, both methods falter in replicating natural fertilization's fidelity, with SCNT's oocyte scarcity and ethical barriers hindering human reproductive applications, while iPSCs offer practicality at the cost of epigenetic fidelity.[55][49]

Empirical Achievements and Evidence

Animal Cloning Outcomes

The first mammal successfully cloned using somatic cell nuclear transfer (SCNT) was Dolly the sheep, born on July 5, 1996, to Scottish researchers at the Roslin Institute. Dolly developed progressive lung disease (ovine pulmonary adenocarcinoma) and arthritis, leading to euthanasia on February 14, 2003, at approximately 6.5 years of age—less than half the typical 11-12 year lifespan for her breed (Finn Dorset).[1] [56] Subsequent analyses of Dolly's telomeres indicated shortened lengths suggestive of accelerated cellular aging, though direct causation remains debated.[57] However, four cloned sheep produced later using similar adult fibroblast cells—Debbie, Denise, Dianna, and Daisy—exhibited normal metabolic and age-related health markers at 7-9 years of age, free from common geriatric conditions like hypertension or diabetes, challenging early concerns of universal premature senescence.[58] [59] SCNT cloning efficiency across species remains low, typically yielding 0-10 live births per 100 transferred embryos, with most failures occurring during embryonic development, implantation, or gestation due to incomplete nuclear reprogramming and epigenetic errors.[34] In ruminants like cattle and sheep, large offspring syndrome (LOS)—characterized by fetal overgrowth, macrosomia, placental abnormalities, and hydroallantois—occurs in a significant proportion of pregnancies, increasing risks of dystocia, respiratory distress, and neonatal mortality.[60] [61] Cloned fetuses often display abnormal organ development, immune deficiencies, and cardiovascular issues, with post-birth survival rates under 5% in early bovine trials.[62] These outcomes stem from faulty gene expression patterns persisting from the donor somatic cell, leading to disrupted imprinting and mitochondrial incompatibilities.[54] Outcomes vary by species but consistently involve high procedural losses. In cattle, cloned since 1998, thousands have been produced for agricultural traits, yet many exhibit LOS-related defects or die perinatally; surviving clones and their offspring generally reach reproductive maturity without elevated disease incidence, per U.S. FDA assessments of 2008-2021 data.[63] Porcine clones, initiated in 2000, face similar placental and organ enlargement issues, with efficiency below 2% and frequent early deaths from heart failure or infections.[64] Mice, cloned via SCNT since 1998, achieve higher relative success (up to 5% in optimized protocols) but suffer high embryonic lethality and adult-onset tumors or obesity in some lines.[65] Dogs, with over 1,500 SCNT clones produced commercially by 2022 across ~20 breeds, demonstrate viable reproduction but incur substantial losses during gestation and neonate stages, mirroring ruminant patterns.[66] In primates, the first SCNT-cloned rhesus macaques, Zhong Zhong and Hua Hua, born in 2018, were over 6 years old as of 2024 and reported healthy, with normal lifespan expectations of 25-40 years in captivity; this indicates partial plausibility for long-term viability in species phylogenetically close to humans, alongside observed normal reproduction in cloned mammals, though inefficiencies, high failure rates, embryonic issues, and health risks such as placental abnormalities persist.[67] [68] Other species, including cats (2001 onward), horses, and rabbits, report comparable inefficiencies and health anomalies, though long-term data indicate many healthy adults when gestation completes successfully.[1] Overall, while clones can thrive, the process's empirical toll—encompassing 90-99% failure rates and welfare impairments—highlights persistent biological barriers to reliable replication.[69]

Human Therapeutic Cloning Advances

In 2013, researchers led by Shoukhrat Mitalipov at Oregon Health & Science University achieved the first successful derivation of human embryonic stem cell (hESC) lines using somatic cell nuclear transfer (SCNT). The team transferred nuclei from human fetal fibroblasts into enucleated oocytes, resulting in the development of blastocysts from which pluripotent hESC lines were isolated; these cells demonstrated normal karyotypes and pluripotency markers comparable to those from fertilized embryos.00571-0) This breakthrough followed optimizations from prior primate SCNT work, addressing previous inefficiencies in human attempts, such as incomplete reprogramming and developmental arrest.[70] Subsequent advances in 2014 involved deriving hESC lines from adult human somatic cells. Independent teams at the New York Stem Cell Foundation, led by Dieter Egli, and in South Korea successfully cloned embryos using adult cumulus cells via refined SCNT protocols, including caffeine treatment to prevent premature activation and histone deacetylase inhibitors to enhance reprogramming. These efforts yielded viable hESC lines genetically identical to the donors, confirming the technique's applicability to adult cells without relying on fetal sources.[71] Despite these milestones, therapeutic cloning via SCNT has not progressed to clinical applications in humans. As of 2020, no human embryos have been produced through SCNT for therapeutic purposes leading to treatments, with research limited by low efficiency rates—often below 5% blastocyst formation—and ethical restrictions on oocyte sourcing. Induced pluripotent stem cells (iPSCs) have largely supplanted SCNT in regenerative medicine due to avoiding embryo creation, though SCNT-derived hESCs offer advantages in mitochondrial compatibility for certain mitochondrial diseases.[1] Ongoing refinements focus on improving yield and safety, but empirical evidence of therapeutic efficacy remains preclinical.[37]

Reproductive Cloning Status

No verified instances of successful human reproductive cloning, defined as the production of a genetically identical human via somatic cell nuclear transfer (SCNT) leading to live birth, have occurred as of 2026.[18] Scientific consensus holds that while cloned human embryos have been created in laboratory settings, none have progressed to viable pregnancies or births due to profound technical challenges and ethical prohibitions.[1] Early claims, such as those by Clonaid in 2002 announcing the birth of a cloned infant named "Eve," lacked independent verification and were dismissed by experts as unsubstantiated or hoaxes, with no DNA evidence provided despite demands.[72] Subsequent attempts, including announcements by figures like Severino Antinori in the early 2000s, failed to produce confirmed results, often ending in embryo transfer without pregnancy confirmation or retraction of claims.[73] In animals, SCNT yields low success rates—typically under 5% live births—and cloned offspring often exhibit high incidences of abnormalities like large offspring syndrome, immune deficiencies, and premature aging. However, recent primate cloning successes, such as the 2018 production of healthy rhesus macaques via SCNT that have survived over 6 years with normal health and lifespan expectations of 25-30 years in captivity, indicate improved feasibility, alongside evidence of normal reproduction in cloned mammals of other species; the process remains inefficient overall, with high failure rates, embryonic issues, and health risks like placental abnormalities, patterns expected to amplify in humans given physiological complexities.[74] Human embryo cloning experiments, such as those reported in 2001 by Advanced Cell Technology reaching four-to-six cell stages, halted short of implantation due to inefficiency and developmental arrest.[6] Legally, reproductive cloning faces near-universal bans or heavy restrictions in most countries due to ethical, safety, and medical concerns, with approximately 46 countries enacting explicit prohibitions, including penalties up to 20 years imprisonment in France as reinforced in 2025.[75][9] International bodies like UNESCO advocate against it, citing risks to human dignity and safety, while bodies such as the U.S. President's Council on Bioethics have recommended indefinite federal bans.[17] These restrictions, coupled with institutional review board oversight and funding limitations, preclude sanctioned research, rendering practical advancement improbable absent regulatory shifts. Despite theoretical feasibility post-2013 embryo cloning demonstrations, no credible post-2020 efforts toward birth have surfaced, underscoring persistent viability barriers over sensational unverified assertions.[24][76]

Potential Benefits and Applications

Medical and Therapeutic Prospects

Therapeutic cloning, primarily through somatic cell nuclear transfer (SCNT), enables the creation of patient-specific embryonic stem cells by replacing the nucleus of an enucleated oocyte with a somatic cell from the patient, followed by activation to form a blastocyst from which stem cells are derived.[77] These cells are genetically identical to the donor, minimizing immune rejection risks in regenerative therapies without requiring lifelong immunosuppression.[78] Potential applications include generating dopaminergic neurons for Parkinson's disease, where animal models using SCNT-derived cells have shown functional restoration of motor deficits.[37] In diabetes treatment, SCNT could produce insulin-secreting beta cells tailored to the patient, addressing the autoimmune destruction of pancreatic islets in type 1 diabetes; preclinical studies in non-human primates demonstrate viability of such approaches for personalized islet transplantation.[79] For spinal cord injuries and myocardial infarction, cloned stem cells might differentiate into neural or cardiac tissue, promoting repair; early human SCNT lines established in 2014 confirmed pluripotency and normal karyotypes, supporting feasibility for tissue engineering.[48] Additionally, these cells facilitate disease modeling and drug screening, accelerating personalized medicine by replicating patient-specific pathologies in vitro.[80] Beyond direct cell replacement, therapeutic cloning prospects extend to organ regeneration, such as bioengineered kidneys or livers from cloned progenitors, potentially alleviating transplant shortages; while human applications remain experimental, successes in cloning porcine organs for xenotransplantation hint at scalable therapeutic paradigms.[81] Integration with genome editing, like CRISPR, could correct underlying genetic defects prior to differentiation, enhancing outcomes for monogenic disorders such as cystic fibrosis or sickle cell anemia.[37] Despite technical hurdles like low efficiency—typically under 5% in mammalian SCNT—these methods offer causal advantages in histocompatibility over allogeneic sources, positioning therapeutic cloning as a cornerstone for future autologous therapies.[51]

Reproductive and Personal Uses

Reproductive cloning involves the creation of a genetically identical human embryo via somatic cell nuclear transfer (SCNT), with the intent to implant it for gestation to term, resulting in a live birth of a cloned individual.[1] This differs from therapeutic cloning by aiming for full human development rather than tissue or organ production. Proponents argue it could enable infertile individuals or couples to produce offspring genetically related to one parent, bypassing limitations of gamete donation or surrogacy where full genetic identity to both is impossible.[82] For instance, a couple where one partner has gamete failure could clone the fertile partner's cells to generate an embryo sharing that parent's full genome, though mitochondrial DNA from the egg donor would introduce minor non-nuclear variation.[2] Personal motivations for reproductive cloning often center on preserving individual genetics across generations or mitigating personal loss. Some envision cloning deceased relatives—such as a child lost to accident or disease—to create a genetically identical sibling, allowing families to "continue" a lineage or grieve through a biological facsimile.[1] Self-cloning has been hypothesized for extending personal legacy, where an individual produces a genetic twin raised in a controlled environment to inherit specific traits or knowledge, though environmental factors like epigenetics and upbringing would prevent true identity replication.[3] Advocates, including bioethicists like Julian Savulescu, contend such applications could fulfill autonomous reproductive rights, akin to existing assisted reproduction technologies, provided safety thresholds are met.[12] Despite these rationales, no verified human reproductive cloning has succeeded as of 2025, with historical claims—such as the 2002 announcement by Clonaid of a cloned infant named Eve—lacking empirical evidence and dismissed by scientific consensus due to absence of genetic verification or peer-reviewed data.[1] Animal models, including Dolly the sheep cloned in 1996, demonstrate feasibility in principle but highlight inefficiencies: success rates below 5% in mammals, with frequent developmental anomalies like large offspring syndrome and premature aging.[83] These empirical hurdles render personal uses speculative, as human gestation lacks the optimizations possible in veterinary cloning, and unverified attempts risk unquantified health defects in clones.[8] Nonetheless, theoretical personal benefits persist in discourse, such as generating immunologically matched siblings for tissue compatibility in families with rare genetic disorders, extending beyond mere reproduction to targeted familial health strategies.[82]

Risks and Empirical Challenges

Biological and Health Hazards

Reproductive cloning via somatic cell nuclear transfer (SCNT) in mammals exhibits profound inefficiencies, with success rates typically below 5%, often requiring hundreds of attempts to produce a viable offspring, as evidenced by the 277 nuclear transfer procedures needed for Dolly the sheep in 1996.[1] This stems from incomplete epigenetic reprogramming of the donor somatic nucleus, leading to aberrant gene expression, disrupted genomic imprinting, and high rates of embryonic lethality or fetal loss.[84] In surviving clones, common outcomes include large offspring syndrome (LOS), characterized by oversized fetuses, placental overgrowth, and cardiovascular strain, which contributes to dystocia and neonatal mortality exceeding 90% in some species like cattle and sheep.[85] Cloned animals frequently suffer organ malformations, immune deficiencies, and metabolic disorders due to these reprogramming failures, with studies in bovine clones reporting elevated incidences of hepatic steatosis, renal dysplasia, and pulmonary hypertension.[3] Long-term health concerns involve potential premature aging linked to telomere attrition, as somatic donor cells carry shortened telomeres from prior divisions, though telomerase reactivation in embryos can partially restore length; however, one-third to half of cloned cohorts show persistent reductions compared to age-matched controls, correlating with accelerated senescence in tissues.[86] While some cloned sheep derived from Dolly's cell line reached ages of 9 years without overt aging deficits, earlier cases like Dolly's euthanasia at age 6 due to progressive lung disease and osteoarthritis fueled debates over cloning-induced vulnerabilities, underscoring unresolved risks of stochastic epigenetic errors.[87] [58] In human therapeutic cloning, SCNT-derived embryos for stem cell production face analogous hazards, including chromosomal instability and oncogenic transformations from faulty reprogramming, with animal models indicating heightened tumor formation risks in derived tissues.[88] Mitochondrial heteroplasmy—carryover of donor oocyte mitochondria mismatched with the nuclear genome—poses additional threats, potentially triggering immune-mediated rejection even in autologous transplants, as demonstrated in murine studies where mitochondrial antigens elicited T-cell responses leading to graft failure.[89] These biological imperatives, rooted in the causal mismatches between differentiated donor nuclei and embryonic contexts, render human cloning hazardous without technological breakthroughs to mitigate pervasive developmental and oncogenic perils.[11]

Efficiency and Viability Issues

Somatic cell nuclear transfer (SCNT), the primary technique for human cloning, exhibits extremely low efficiency in mammalian species, with live birth rates typically ranging from 1% to 5% of transferred embryos in animals such as mice, sheep, and cattle.[51] This inefficiency arises from incomplete epigenetic reprogramming of the donor nucleus, leading to failures in gene expression and developmental arrest at early embryonic stages.[34] In sheep cloning, blastocyst formation rates reach 5.3% to 42%, but progression to viable newborns occurs in only 5.7% to 15% of transferred embryos, underscoring persistent technical barriers.[90] For prospective human reproductive cloning, these animal outcomes indicate profound viability challenges, as no verified live births have occurred despite claims, with efficiency likely even lower due to human oocyte scarcity and heightened ethical constraints on experimentation.[1] Cloned animals frequently suffer from severe health defects, including large offspring syndrome characterized by oversized fetuses, placental abnormalities, and respiratory distress, contributing to high perinatal mortality rates—up to 42% in cloned calves within the first 150 days.[91][85] Epigenetic errors, such as improper DNA methylation and histone modifications, causally underlie these issues, resulting in organ dysfunction, immune deficiencies, and accelerated aging in survivors.[92] Therapeutic cloning via SCNT for human embryonic stem cell derivation faces analogous hurdles, with blastocyst development rates historically below 10% and derivation of viable stem cell lines rare until isolated advances, such as the 2013 production of human nuclear transfer embryonic stem cells from adult fibroblasts at efficiencies not exceeding a few percent.[93][94] While some cloned animals, like dogs and sheep, show no evident long-term health deficits post-survival, the overall process demands hundreds of oocyte donations and embryo transfers per success, rendering it economically and biologically unviable for routine application without major reprogramming breakthroughs.[66][95] These empirical limitations highlight SCNT's fundamental constraints in achieving reliable nuclear totipotency.[96]

Ethical and Philosophical Debates

Arguments in Favor of Cloning

Proponents of human reproductive cloning contend that it could enable infertile individuals or couples to have genetically related offspring, extending reproductive technologies beyond gamete donation or surrogacy to produce a child with one parent's nuclear DNA.[97] This approach has been proposed as a means to fulfill the desire for biological continuity in cases where traditional conception fails due to sterility or gamete scarcity, akin to how in vitro fertilization addressed prior barriers.[82] Another argument posits that cloning offers a way to mitigate certain inherited genetic disorders by replicating the genome of a healthy parent, thereby avoiding the recombination risks in sexual reproduction that can propagate recessive lethal alleles. For instance, if both parents carry a single copy of a recessive disease gene, natural procreation yields a 25% chance of an affected child, whereas cloning the unaffected parent's genome eliminates this probability.[98] Advocates emphasize that this method preserves familial genetic lineage without introducing foreign donor DNA, potentially reducing the incidence of conditions like cystic fibrosis or Tay-Sachs disease in high-risk pedigrees.[99] Cloning has also been advocated to recreate a deceased loved one, such as a child lost to accident or illness, providing parents an opportunity to raise a genetic duplicate and thereby alleviate profound grief through renewed parental bonds.[99] This perspective frames cloning not as identical resurrection but as a biological proxy that honors the original's genome while allowing for environmental influences to shape a distinct individual.[97] From a utilitarian standpoint, some scholars suggest human cloning could enhance human capabilities by duplicating genomes associated with exceptional health, intelligence, or achievement, thereby accelerating societal progress without relying solely on random genetic variation.[12] This includes selecting for traits like disease resistance or cognitive prowess, which proponents argue would yield net benefits in population-level fitness if technical efficiencies improve beyond current animal cloning success rates of under 5% viable births.[100] Such applications are viewed as extensions of selective breeding principles observed in agriculture and animal husbandry, applied ethically to voluntary human enhancement.[12]

Arguments Against Cloning

Opponents of human cloning, particularly reproductive cloning, argue that it undermines human dignity by treating individuals as manufactured copies rather than unique beings arising from natural procreation. The United Nations General Assembly's 2005 Declaration on Human Cloning urged member states to prohibit all forms of human cloning, deeming them incompatible with human dignity and the protection of human life.[101] This position reflects concerns that cloning instrumentalizes human life, reducing persons to genetic replicas for parental or societal purposes, thereby eroding the intrinsic value of individuality.[17] Empirical data from animal cloning underscores profound biological risks, including high rates of embryonic failure, gestational abnormalities, and post-birth health defects, which would likely translate to human attempts. Reproductive cloning in mammals is inefficient, with success rates below 5% in species like sheep and cattle; for instance, Dolly the sheep required 277 nuclear transfer attempts, and most cloned embryos fail to implant or develop normally. Surviving clones often exhibit large offspring syndrome, organ enlargement, immune deficiencies, and premature aging due to incomplete epigenetic reprogramming, as evidenced by shortened telomeres and elevated disease incidence in bovine and ovine clones.[102] Dolly developed arthritis at age 5—unusual for her breed—and was euthanized at 6.5 years due to progressive lung disease, prompting early concerns about accelerated aging in clones, though subsequent clones from the same cell line showed no such anomalies, indicating persistent technical uncertainties rather than resolved risks.[103] Such biological hazards extend to surrogate mothers, who face elevated risks of miscarriage, placental abnormalities, and complications from oversized fetuses, as documented in cloned animal pregnancies with abortion rates exceeding 50%.[61] The National Academy of Sciences has warned that human reproductive cloning would inevitably produce suffering through trial-and-error iterations, with many clones dying in utero or shortly after birth, and survivors prone to unforeseen pathologies absent in natural reproduction.[104] Social and psychological arguments highlight potential harms to cloned individuals, including identity crises from predetermined genetic origins and societal perceptions of them as replacements or experiments, akin to but exceeding challenges faced by twins.[82] Clones may experience reduced autonomy if viewed through the lens of their progenitor's expectations, fostering psychological distress or exploitation, as ethicists contend that replication denies the open-endedness of human development.[12] Broader societal risks include exacerbating inequalities, as cloning access would favor the wealthy, potentially leading to a stratified class of "designer" humans and reinforcing eugenic pressures without addressing underlying genetic diversities that confer resilience.[98] These concerns, grounded in observed animal outcomes and philosophical analysis of personhood, substantiate calls for indefinite prohibition to avert irreversible ethical and empirical pitfalls.

Debunking Prevalent Misconceptions

A prevalent misconception holds that human reproductive cloning has already been successfully achieved, often fueled by unsubstantiated claims such as those from the Clonaid organization in 2002, which announced the birth of a cloned infant named "Eve" without providing verifiable evidence or allowing independent verification. No peer-reviewed scientific confirmation of a live-born human clone via somatic cell nuclear transfer exists, and experts attribute the absence of verified successes to persistent technical barriers, including low implantation rates and high embryonic loss observed in mammalian cloning experiments.[24][3] Another widespread myth posits that clones would be exact replicas of the donor in every respect, including physical appearance, personality, and aging trajectory, akin to a photocopy. In reality, while nuclear DNA would match the somatic cell donor, mitochondrial DNA derives from the egg donor, introducing genetic variation; moreover, epigenetic modifications, environmental influences, and stochastic developmental events produce differences comparable to those between identical twins raised apart. Cloned animals exhibit phenotypic variations from their donors, underscoring that cloning replicates genetic starting material but not life experiences or non-genetic factors.[105][106] Concerns about premature aging in clones stem largely from Dolly the sheep's early death in 2003 at age 6.5 years from a lung infection, compounded by her shortened telomeres and arthritis, which prompted fears of inherent replicative senescence in clones. However, a 2016 study of four sheep cloned from the same cell line as Dolly revealed normal lifespans of 7 to 9.3 years with no unanticipated health deficits, and telomere restoration techniques, such as telomerase activation, have yielded clones with telomere lengths equivalent to age-matched controls. Dolly's joint wear was later deemed comparable to non-clones of similar breed and obesity, indicating her issues were not uniquely attributable to cloning. Ongoing refinements in nuclear transfer protocols have reduced such anomalies, with many cloned mammals achieving health outcomes indistinguishable from conventionally bred counterparts.[95][107][108] The belief that cloning is a straightforward, high-yield process ignores empirical data from animal trials, where success rates remain below 5-10% for live births, often involving hundreds of reconstructed embryos, frequent miscarriages, and neonatal defects like large offspring syndrome. Techniques have improved since Dolly's 1996 creation—achieved after 277 attempts—but human applications face amplified challenges due to protracted gestation and ethical constraints on large-scale trials, rendering current claims of feasibility overstated.[109][110]

Legal and Regulatory Frameworks

International Positions

International regulatory efforts on human cloning have produced non-binding declarations and limited regional protocols, with no comprehensive global treaty in force. The United Nations General Assembly adopted the United Nations Declaration on Human Cloning on March 8, 2005, calling on member states to prohibit all forms of human cloning on grounds that such practices are incompatible with human dignity and the protection of human life in its dignity and integrity. The resolution passed by a vote of 84 in favor, 34 against, and 37 abstentions, highlighting divisions particularly over therapeutic applications of cloning techniques for biomedical research.[101][111] The Council of Europe established the first multilateral instrument specifically targeting cloning through the Additional Protocol to the Convention for the Protection of Human Rights and Dignity of the Human Being with regard to the Application of Biology and Medicine, opened for signature on January 12, 1998. Article 1 of the protocol states: "Any intervention seeking to create a human being genetically identical to another human being, whether living or dead, is prohibited," applying to interventions performed in the signatory state or by its nationals abroad. Ratified by 23 countries as of 2023, the protocol emphasizes reproductive cloning but leaves therapeutic uses unaddressed, serving as a model for national legislation in Europe.[112] UNESCO's Universal Declaration on the Human Genome and Human Rights, adopted by the General Conference on November 11, 1997, declares that "practices which are contrary to human dignity, such as reproductive cloning of human beings, shall not be permitted," underscoring the need for international bioethical standards to safeguard genomic integrity. Subsequent UNESCO reports, including those from the International Bioethics Committee, have reinforced opposition to reproductive cloning while advocating for governance frameworks that distinguish it from research-oriented somatic cell nuclear transfer.[113][114] The World Health Organization has consistently advised against human reproductive cloning, citing empirical evidence from animal studies showing high rates of developmental abnormalities, implantation failures, and health risks in clones, such as large offspring syndrome and premature aging. In a 2004 report to the Executive Board, WHO noted the absence of safe, verified techniques for human application and recommended global vigilance without endorsing prohibitions on therapeutic cloning for stem cell research. These positions reflect a consensus against reproductive cloning due to verifiable biological hazards, though enforcement remains dependent on national laws amid varying interpretations of dignity and scientific utility.[115]

Domestic Laws by Jurisdiction

Domestic laws on human cloning predominantly prohibit reproductive cloning, aimed at producing a cloned human being, while therapeutic cloning—for deriving embryonic stem cells—receives more varied treatment, with permissions in some nations under strict oversight. Over 46 countries have enacted formal bans on reproductive cloning, though enforcement and scope differ.[9] Bans typically criminalize the transfer of a cloned embryo to a uterus, with penalties including imprisonment. In the United States, no federal statute explicitly prohibits human reproductive cloning, leaving it unregulated at the national level beyond restrictions on federal funding for cloning research.[116] Several states have imposed bans: California, Iowa, Louisiana, Michigan, Rhode Island, and Virginia prohibit both reproductive and therapeutic cloning, while others like Arizona and Florida ban only reproductive efforts.[117] Therapeutic cloning remains permissible in states without specific prohibitions, subject to institutional review boards and ethical guidelines from bodies like the National Academies.[116] Canada's Assisted Human Reproduction Act of 2004 explicitly bans human cloning, including the creation of cloned embryos for any purpose, with penalties up to 10 years imprisonment and fines.[118] This encompasses both reproductive and therapeutic applications, prohibiting embryo cloning for stem cell derivation.[9] In the United Kingdom, the Human Reproductive Cloning Act 2001 criminalizes reproductive cloning, making it an offense to place a cloned embryo in a woman, punishable by up to 10 years in prison.[119] Therapeutic cloning is permitted under licenses from the Human Fertilisation and Embryology Authority, allowing somatic cell nuclear transfer for research but not implantation.[120] Australia's Prohibition of Human Cloning for Reproduction Act 2002 bans reproductive cloning nationwide, with 15-year prison terms for violations, and also prohibits therapeutic cloning involving embryo creation for research beyond 14 days.[121] States like Victoria permit certain stem cell research but align with federal bans on cloning.[118] Japan's 2001 guidelines, updated in subsequent laws, prohibit human reproductive cloning as unethical, with criminal penalties, but authorize therapeutic cloning for medical research under the Ministry of Education, Culture, Sports, Science and Technology oversight.[122] China bans reproductive cloning through regulations from the Ministry of Science and Technology, emphasizing ethical guidelines post-2004, but permits therapeutic cloning and embryo research for biomedical purposes, as seen in approvals for stem cell derivation.[72][123] Across European Union member states, reproductive cloning is uniformly prohibited under national bioethics laws, often aligned with the Council of Europe's 1998 Oviedo Protocol, ratified by 23 nations, which bans any cloning creating human beings.[124] France's 2025 bioethics law reinforces a ban on reproductive cloning with up to 20 years imprisonment, while therapeutic cloning remains restricted.[75] Germany and Austria extend bans to all forms of cloning, including therapeutic embryo creation. Variations persist, with some like Belgium allowing limited therapeutic research.[9]
JurisdictionReproductive CloningTherapeutic CloningKey Legislation
United States (Federal)Not bannedFunding restricted; state variationsNo comprehensive federal law[116]
CanadaBannedBannedAssisted Human Reproduction Act 2004[118]
United KingdomBannedPermitted with licenseHuman Reproductive Cloning Act 2001[119]
AustraliaBannedRestricted (no embryo >14 days)Prohibition of Human Cloning Act 2002[121]
JapanBannedPermitted2001 Guidelines
ChinaBannedPermitted for researchMinistry Regulations 2004[72]
FranceBanned (20 years max)RestrictedBioethics Law 2025[75]
GermanyBanned (all forms)BannedEmbryo Protection Act

Societal and Future Implications

Cultural and Psychological Dimensions

In popular culture, human cloning is frequently depicted as a perilous endeavor that erodes individuality and invites societal collapse, drawing from literary and cinematic tropes such as Mary Shelley's Frankenstein and films like Blade Runner, where clones are portrayed as tormented replicas lacking authentic humanity or as tools for eugenic control.[125] These representations, amplified by documentaries emphasizing metaphors of scientific hubris like "dyke breaking," foster public anxieties about cloning as a violation of natural order, often conflating it with dystopian outcomes rather than empirical realities.[125] Such narratives, rooted in mythic archetypes of Prometheus or the Golem, have culturally entrenched cloning as taboo, influencing opposition beyond scientific discourse.[125] Religiously, human cloning encounters broad resistance, particularly within Christianity, where it is viewed as usurping divine sovereignty over life creation, as humans are deemed uniquely formed in God's image (Genesis 1:26-27), rendering asexual replication a commodification of sacred personhood.[126] Catholic doctrine specifically condemns it for destroying embryos and undermining procreation's relational essence, while conservative Protestants cite risks to human dignity akin to rejecting the Incarnation.[127] Jewish perspectives vary, with some permitting therapeutic applications on existing embryos but opposing reproductive cloning due to eugenic perils and social disruptions, and Islamic views highlight threats to family lineage while allowing limited research on surplus IVF embryos, tied to ensoulment at around 40 days.[127] These stances reflect causal concerns over cloning's potential to alter kinship structures and equality before divine or natural law, with surveys indicating 81% of Americans opposing unrestricted cloning, often motivated by faith-based qualms.[127] Psychologically, human reproductive cloning raises questions of identity formation, yet empirical analogies from monozygotic (MZ) twins—natural genetic duplicates—indicate no inherent impairment, as twins routinely develop distinct personalities through environmental divergences, challenging assumptions of predestined sameness.[128] Studies like the Minnesota Twin Study affirm MZ individuals' resilience and individuality, suggesting clones would similarly forge unique self-concepts despite genetic origins, without elevated mental disorder risks.[8] However, cultural stigma and parental projections could impose burdens, such as reduced empathy from progenitors presuming innate understanding, potentially fostering identity confusion if the clone is treated as a proxy for the donor.[128] Family dynamics may further complicate outcomes, with cloned children risking objectification as "replacements" or extensions of parental desires, straining autonomy and self-worth, though twin data counters fears of inevitable psychological harm by demonstrating adaptive differentiation.[129] Societal perceptions, amplified by media, could exacerbate these via expectations of uniformity, yet first-principles reasoning from developmental psychology underscores that nurture overrides genetic determinism in shaping psyche, implying clones' viability hinges more on rearing than replication itself.[130] Overall, while hypothetical, these dimensions highlight cloning's potential to disrupt conventional notions of uniqueness without empirically dooming the cloned to pathology.[8]

Prospects for Technological Advancement

Recent advances in somatic cell nuclear transfer (SCNT) have focused on overcoming epigenetic barriers that hinder reprogramming efficiency, with a 2025 study demonstrating that overexpression of the Kdm4d gene in mouse SCNT embryos improved pre-implantation development and raised cloning success rates to approximately 8%, compared to typical rates below 5% in prior methods.[131] Similar epigenetic interventions, such as histone deacetylase inhibitors and cyclin-dependent kinase modulators, have enhanced embryonic viability in mice by addressing incomplete nuclear reprogramming inherited from donor somatic cells.[132] In primates, successful SCNT cloning of cynomolgus monkeys, including gene-edited variants, was reported in 2024, marking progress toward viable reproductive cloning in species closer to humans, though live birth rates remain under 1%.[133] Despite these gains, mammalian SCNT faces persistent challenges, including low overall efficiency (often 1-3% live births in livestock), high rates of developmental arrest, and post-natal abnormalities like large offspring syndrome, organ dysfunction, and premature aging due to faulty genomic imprinting and mitochondrial incompatibilities.[88] In non-human primates, cloning is particularly inefficient owing to species-specific proteins that regulate embryonic genome activation, exacerbating reprogramming failures compared to rodents or ungulates.[1] Interspecies SCNT experiments, reviewed through 2023, highlight additional barriers like nuclear-cytoplasmic mismatches, but also suggest potential for hybrid approaches to test human-relevant protocols without direct human embryo use.[134] For human reproductive cloning, technical feasibility hinges on extrapolating from animal models, yet no verified successes exist as of 2025, with prospects tempered by amplified risks: projected failure rates exceeding 99% based on primate data, ethical constraints on large-scale embryo experimentation, and unaddressed human-specific epigenetic hurdles.[135] Improvements in second-generation cloning, as shown in 2025 sheep studies yielding viable transgenic offspring from cloned donors, indicate iterative refinement could mitigate cumulative errors, potentially enabling human applications within decades if regulatory barriers lift.[136] Therapeutic cloning via SCNT for patient-specific stem cells has diminished in prominence, overshadowed by induced pluripotent stem cells (iPSCs) since their 2006 derivation, which bypass egg donation and ethical issues while achieving comparable pluripotency without tumorigenic vectors in refined protocols.[42] iPSCs now dominate regenerative medicine pipelines, with clinical trials advancing for conditions like macular degeneration by 2024, rendering SCNT less viable due to lower efficiency and higher costs, though SCNT may retain niche utility for precise mitochondrial-nuclear matching.[43] Future breakthroughs, such as CRISPR-based epigenetic editing integrated with SCNT, could revitalize therapeutic prospects by resolving heteroplasmy and immunogenicity, but empirical data prioritize iPSC scalability over cloning's complexities.[49]

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