Origin recognition complex

The origin recognition complex (ORC) is a highly conserved, multi-subunit protein complex that serves as the initiator of eukaryotic DNA replication by binding to origins of replication and assembling the pre-replicative complex (pre-RC).^[1] Composed of six subunits (Orc1–Orc6), ORC was first identified in the budding yeast Saccharomyces cerevisiae approximately 30 years ago and plays a central role in marking replication start sites, recruiting Cdc6 and Cdt1 to load the MCM2–7 helicase double hexamer during G1 phase of the cell cycle.^[1][2] This process ensures once-per-cell-cycle replication, preventing re-replication through cell cycle-regulated mechanisms such as cyclin-dependent kinase (CDK) phosphorylation.^[1] Structurally, ORC forms a clamp-like architecture, with Orc1–5 adopting AAA+ ATPase domains that facilitate ATP-dependent conformational changes essential for helicase loading, while Orc6 contributes to DNA binding and complex stability but is less conserved across species.^[3] In S. cerevisiae, ORC exhibits sequence-specific binding to the ARS consensus sequence (ACS) via specialized motifs in Orc1, Orc2, and Orc4, whereas in metazoans like humans, binding is more flexible, often influenced by nucleosome positioning, chromatin accessibility, and AT-rich regions rather than strict sequence motifs.^[1][3] Cryo-electron microscopy (cryoEM) studies have revealed dynamic conformational states of human ORC, highlighting its adaptability in origin recognition and its differences from yeast ORC, such as the absence of sequence specificity and reliance on post-translational modifications for regulation.^[3] Evolutionarily, ORC subunits show deep conservation from yeast to humans, with Orc1–5 sharing AAA+ and winged-helix domains reminiscent of archaeal ORC-like proteins and bacterial DnaA, suggesting an ancient origin tied to genome duplication events.^[2] Variations in subunit number and function across eukaryotes reflect adaptations to diverse genome sizes and replication needs, including roles beyond replication such as heterochromatin organization and centromere function in some yeasts. Recent studies as of 2025 have further shown that in human cells, ORC, particularly the Orc2 subunit, regulates epigenetics, gene expression, and chromosome structure.^[1][4] Dysfunctions in ORC, particularly mutations in ORC1, are linked to developmental disorders like Meier-Gorlin syndrome, underscoring its critical physiological importance.^[3]

Composition and Structure

Protein Subunits

The origin recognition complex (ORC) is composed of six conserved subunits, designated Orc1 through Orc6, which were originally identified and named in budding yeast (Saccharomyces cerevisiae) based on their approximate molecular masses.^[5] These subunits assemble into a heterohexameric structure essential for DNA replication initiation, with each contributing distinct domains and functions while exhibiting varying degrees of sequence conservation across eukaryotes.^[6] Orc1 is the largest subunit, with a molecular weight of approximately 100 kDa in yeast and around 110 kDa in humans.^[7] It contains an N-terminal bromo-adjacent homology (BAH) domain for chromatin interactions, a central AAA+ ATPase domain, and a C-terminal winged-helix domain for DNA binding.^[5] Orc1 exhibits ATPase activity critical for origin recognition and serves as a key DNA-binding component, while also facilitating the recruitment of other replication factors.^[6] Sequence conservation of Orc1 is high across eukaryotes, reflecting its fundamental role in replication licensing.^[5] Orc2 and Orc3 form a stable core within the complex, with molecular weights of about 70 kDa and 70 kDa in yeast, respectively, and similar sizes (~70 kDa and ~80 kDa) in humans.^[8][9] Both subunits possess AAA+ ATPase domains and winged-helix motifs that contribute to DNA binding and overall complex stability, though Orc3 lacks strong direct DNA-binding affinity on its own.^[6] Orc2 supports mitotic progression and core assembly, while Orc3 reinforces the structural integrity of Orc1–Orc5.^[5] These core subunits show high sequence similarity across eukaryotic species, underscoring their conserved architectural role.^[5] Orc4 has a molecular weight of approximately 50–60 kDa in both yeast and humans and includes an AAA+ domain and a winged-helix fold.^[10] It contributes to DNA binding through AT-hook-like motifs in some species and provides an arginine finger residue essential for stimulating Orc1's ATPase activity.^[6] Orc4 also exhibits features resembling histone interactions, aiding in chromatin association at origins.^[5] Conservation is strong for Orc4, particularly in its ATPase-related regions, though metazoan variants include species-specific insertions for origin specificity.^[6] Orc5, at around 50 kDa in yeast and humans, features an AAA+ ATPase domain and a winged-helix domain that support ATP binding and DNA interactions.^[11] It acts as a sensor for nucleotide states within the complex, helping regulate ATPase cycles and maintaining core stability.^[6] Orc5 is highly conserved across eukaryotes, with essential contributions to pre-replicative complex formation.^[5] Orc6 is the smallest subunit, with a molecular weight of approximately 50 kDa in yeast and ~28 kDa in humans, lacking an AAA+ domain but containing a unique conserved region homologous to transcription factors like TFIIB in metazoans.^[12] It serves as a platform for recruiting Cdt1 and the MCM2-7 helicase during replication initiation and contributes to cytokinesis in some organisms.^[13] Unlike the other subunits, Orc6 displays greater sequence divergence across eukaryotes, with variable roles in DNA binding between yeast and metazoans.^[5]

Architectural Features

The origin recognition complex (ORC) exhibits a conserved hexameric architecture characterized by a ring-like structure that encircles DNA, as revealed by high-resolution cryo-EM studies. The core of the complex, formed by subunits Orc1 through Orc5, adopts a clamp-like conformation with a central channel approximately 30-35 Å in diameter, sufficient to accommodate the DNA double helix. This ring is assembled in a double-layered manner, with the bottom layer comprising AAA+ ATPase domains and the top layer featuring winged-helix domains (WHDs) that contribute to structural stability and DNA interaction.^[14][15] Orc6 attaches peripherally to the Orc1-5 core, often positioned at the base near Orc2 and Orc3, without directly participating in the central ring but stabilizing the overall assembly through its TFIIB-like folds. The AAA+ modules are prominently featured in Orc1, Orc4, and Orc5, enabling ATP binding and hydrolysis that underpin the complex's dynamics, while WHDs in multiple subunits form a spiral arrangement around the periphery. A key structural interface is the Orc2-Orc3 heterodimer, which serves as the foundational core with extensive buried surface area (over 3,000 Ų), anchoring the other subunits and providing rigidity to the clamp.^[14][15][3] Conformational flexibility is integral to ORC's architecture, with structures capturing open and closed states that reflect transitions during assembly and DNA engagement. In the open state, the ring gapes at the Orc1-Orc2 interface, facilitating DNA entry into the central channel, whereas the closed state involves compaction via WHD collapse and hinge motions at the Orc3-Orc5 junction. These dynamics, observed in resolutions ranging from 3.2 to 4.3 Å, highlight the complex's adaptability while maintaining a corkscrew-like twist for efficient DNA clamping. Recent cryo-EM analyses from 2020 to 2024, including those of human and yeast ORC, underscore this modular design's role in forming a functional platform.^[3][16][17]

Origin Recognition and Binding

In Budding Yeast

In the budding yeast Saccharomyces cerevisiae, the origin recognition complex (ORC) binds to autonomously replicating sequences (ARSs), which serve as replication origins. These ARS elements are modular DNA sequences typically spanning 100-150 base pairs and consisting of an essential ARS consensus sequence (ACS) along with auxiliary elements B1, B2, and B3 that enhance origin efficiency.^[18][19] The ACS is an 11-base-pair AT-rich motif with the consensus 5'-(A/T)TTTAT(A/G)TTT(A/T)-3' (where the first, last, and variable positions are as indicated), which can extend to 17 bp in some contexts and is indispensable for origin function.^[19][20] The B1 and B3 elements often serve as binding sites for the transcription factor ABF1, while the B2 element facilitates additional ORC contacts.^[18] The S. cerevisiae genome contains approximately 500 confirmed replication origins, though recent analyses suggest up to 1,600 potential sites.^[21] ORC binds specifically to the ACS via subunits Orc1 and Orc4, with Orc4's basic region recognizing the AT-rich motif and Orc1 contributing through its BAH domain interactions.^[22] This binding is sequence-specific and ATP-dependent, as demonstrated in pioneering studies from the Stillman laboratory during the 1980s and 1990s that identified ORC as the key ARS-binding factor.^[23] ORC remains associated with ARS elements throughout the cell cycle and facilitates pre-replicative complex assembly during G1 phase.^[5] Experimental evidence for ORC-ARS interactions includes in vitro reconstitution assays showing ATP-stimulated binding of purified ORC to ARS1 DNA, confirming the complex's role in origin recognition.^[23] Additionally, chromatin immunoprecipitation (ChIP) studies have mapped ORC occupancy directly to ARS1 and other origins in vivo, revealing strong enrichment at the ACS and adjacent elements.^[24] These findings underscore the precise, sequence-driven mechanism of origin selection in budding yeast.

In Metazoans

In metazoans, the origin recognition complex (ORC) exhibits markedly reduced sequence specificity in binding to replication origins compared to the rigid, ARS consensus sequence-driven mechanism observed in budding yeast. Instead, metazoan origins are primarily defined by chromatin architecture and epigenetic features, allowing for flexible and context-dependent initiation sites across large genomes. This adaptability supports the replication of complex eukaryotic chromosomes, where origins are often clustered in initiation zones spanning tens of kilobases with low individual firing efficiency, typically less than 10%.^[25] Recent studies have shown that the intrinsically disordered region of Orc1 is necessary for ORC recruitment to chromatin in species like Drosophila melanogaster, contributing to sequence-independent binding.^[26] Replication origins in metazoans, such as those in the human genome, number approximately 50,000 potential sites, frequently located at CpG islands, gene promoters, transcriptional insulators, or GC-rich regions that facilitate accessible chromatin. ORC binding relies heavily on nucleosome positioning and histone modifications; for instance, open chromatin marked by histone variant H3.3 and depleted of bulk nucleosomes correlates with ORC occupancy, while active marks like H3K4me3 and H3K9ac are enriched at early-firing origins. The BAH domain of Orc1 specifically recognizes dimethylated histone H4 at lysine 20 (H4K20me2), a modification abundant at licensed origins, thereby anchoring ORC to chromatin and promoting pre-replicative complex stability—a feature conserved across diverse metazoan ORC1 proteins. This histone interaction underscores the epigenetic regulation of origin selection, distinct from sequence motifs.^[27][25][28] In species like Drosophila melanogaster, ORC binds to ACS-like elements at specific loci, such as the chorion gene amplification origins (e.g., ori-β and ACE3), but overall shows broader specificity tied to open chromatin rather than a strict consensus sequence. Human ORC displays even greater plasticity, with binding sites influenced by local transcription and chromatin accessibility, as evidenced by recent studies highlighting origin usage variability under replication stress. Genome-wide ORC-ChIP-seq analyses have mapped thousands of binding sites in human cells, revealing dynamic, cell-type-specific patterns where ORC occupancy correlates with replication timing and can shift between cell states or in response to environmental cues. These approaches, combined with nascent strand sequencing, demonstrate that metazoan origins are stochastically activated, ensuring robust genome duplication.^[29][30][25]

Role in DNA Replication

Pre-Replicative Complex Assembly

The assembly of the pre-replicative complex (pre-RC) initiates with the origin recognition complex (ORC), a heterohexameric protein composed of Orc1–6 subunits, binding to replication origins during the G1 phase of the cell cycle, thereby establishing a foundational platform for subsequent factor recruitment. This binding occurs independently of sequence specificity in metazoans but relies on chromatin interactions, nucleating the ordered addition of Cdc6 and Cdt1. Cdc6 associates with ORC-bound DNA in an ATP-dependent manner, forming an ORC–Cdc6 intermediate that recruits Cdt1-bound MCM2-7 hexamers; Cdt1 acts as a delivery chaperone, positioning the MCM complex onto the DNA for encircling. This sequential process culminates in the loading of two MCM2-7 hexamers in a head-to-head configuration to form a double hexamer (DH), which encircles duplex DNA and licenses the origin for replication.^[31][32][33] Key molecular interactions drive this recruitment and loading. ATP binding by Orc1 enables initial ORC–DNA clamping, while coordinated ATP hydrolysis between Orc1 and Cdc6—occurring first at Cdc6—facilitates stable association and the initial recruitment of Cdt1–MCM2-7, preventing premature dissociation and allowing reiterative loading events. Cdt1 interdigitates between MCM subunits to stabilize delivery, and subsequent ATP hydrolysis primarily by the MCM complex itself powers the closure of the double hexamer around DNA, as visualized in recent cryo-electron microscopy studies of human proteins. These interactions ensure the structural integrity of the OCCM (ORC–Cdc6–Cdt1–MCM) intermediate, which transitions to the mature pre-RC upon Cdt1 release and Cdc6 disengagement. A 2024 structural analysis further revealed that ORC1–5, in conjunction with Cdc6 and Cdt1, assembles the human MCM DH through distinct pathways influenced by ORC6 and Orc1's intrinsically disordered region, highlighting conserved yet species-specific mechanics.^[34][32][32] Stoichiometrically, one ORC per origin licenses a single MCM DH, comprising two MCM2-7 hexamers loaded in a concerted manner, which orients the helicases oppositely on the DNA strands to enable bidirectional replication fork establishment upon S-phase activation. Experimental reconstitutions typically employ equimolar or excess Cdc6 and Cdt1 relative to ORC (e.g., 1:1.5–2 ratios) to achieve efficient DH formation, protecting approximately 55 base pairs of DNA in the final structure. This precise stoichiometry underscores ORC's role as an efficient loader, capable of directing multiple hexamer assemblies without dissociation.^[33][32] To safeguard genomic stability, pre-RC assembly is temporally confined to G1 phase, where low cyclin-dependent kinase (CDK) activity permits ORC-mediated licensing; post-G1 elevation of CDKs phosphorylates ORC components and promotes Orc1 degradation, inhibiting new pre-RC formation and thereby preventing re-replication within the same cell cycle. This checkpoint mechanism ensures origins are licensed exactly once per division, with disruptions leading to replication stress or arrest.^[35][31]

Activation and MCM Loading

The activation of the pre-replicative complex (pre-RC) for DNA replication initiation involves phosphorylation by cyclin-dependent kinase (CDK) and Dbf4-dependent kinase (DDK), which recruit additional factors to the loaded MCM2-7 double hexamer (DH) after ORC release from the origin. DDK first phosphorylates MCM2-7 subunits, such as MCM2, MCM4, and MCM6, to promote helicase activation and association with Cdc45 and GINS, forming the CMG complex essential for replication fork progression. CDK subsequently phosphorylates multiple pre-RC components, including Sld2 and Sld3 in yeast, to further drive origin firing. ORC release post-loading prevents rebinding and re-licensing at the same origin.^[36][37][38] The MCM2-7 DH is loaded in a head-to-head orientation by the ORC-Cdc6-Cdt1 complex, encircling double-stranded DNA as an inactive helicase that requires subsequent activation for unwinding.^[32] This loading process is ATP-dependent, with hydrolysis facilitating the closure of the MCM ring and release of loader components, ensuring stable DH deposition at origins.^[39] Recent biochemical reconstitution in human systems confirms that ORC-Cdc6-Cdt1 efficiently loads two MCM hexamers, forming a tilted interface that positions the helicase for activation.^[32] A key regulatory step is the loading-dependent release of ORC from origins, which occurs after MCM2-7 DH deposition and ensures single-round licensing to avoid re-replication. In yeast, this mechanism displaces ORC from high-efficiency origins during G1 phase, as evidenced by ChIP-seq showing ORC footprints shrinking post-loading, thereby preventing multiple DH assemblies at the same site.^[40] This release is tied to MCM occupancy, with ~66% of origins exhibiting a single DH that overlaps and blocks ORC rebinding, maintaining licensing fidelity across the genome.^[40] Structurally, activation involves conformational shifts in the MCM DH triggered by kinase phosphorylation; cryo-EM structures of the human OCCM intermediate reveal ORC1-5 adopting a C-shaped form that rotates upon Cdc6 binding, inserting DNA into the MCM ring before hydrolysis-driven release of Cdc6 and Cdt1.^[39] In human MCM loading, ORC6 modulates these shifts, enhancing second hexamer recruitment post-hydrolysis, while DDK phosphorylation stabilizes the DH for CMG assembly.^[32] These dynamics, resolved at 3.1 Å resolution, highlight how kinase action propagates through the pre-RC to activate the helicase.^[32] ORC release enables its recycling to license multiple origins, distributing across the genome to load excess MCM2-7 for backup sites. This excess licensing supports dormant origins, which remain inactive during normal S phase but fire under replication stress to maintain fork progression and genome stability. For instance, reducing MCM loading via RNAi impairs dormant origin activation, leading to slowed DNA synthesis and decreased cell viability upon fork stalling.^[41]

Regulation and Evolution

Cell Cycle Regulation

The origin recognition complex (ORC) maintains constitutive association with chromatin throughout the cell cycle in many eukaryotic systems, yet its activity is temporally restricted to the G1 phase to ensure replication occurs only once per cycle. In budding yeast, all six ORC subunits remain bound to replication origins across all phases, providing a stable platform for pre-replicative complex (pre-RC) assembly exclusively during G1 when cyclin-dependent kinase (CDK) activity is low. In metazoans, the core ORC2–ORC6 subcomplex exhibits similar stable chromatin binding, while ORC1 dynamics confer G1 specificity: ORC1 is imported into the nucleus during G1 via its nuclear localization signal and associates with chromatin to activate ORC, but is exported to the cytoplasm or degraded during S phase through cyclin A/CDK2-mediated phosphorylation at multiple sites, preventing untimely licensing.^[42][43][44] In S and G2 phases, multiple inhibitory mechanisms exclude ORC activity to block re-replication. In metazoans, geminin accumulates during S/G2 and inhibits Cdt1, thereby preventing MCM helicase reloading onto ORC-bound origins without directly dissociating ORC from chromatin. Complementarily, CDK phosphorylation of ORC2 and ORC3 subunits disrupts ORC-chromatin interactions, promoting ORC dissociation or inhibiting re-binding to newly replicated DNA; for instance, CDK1/cyclin A phosphorylates ORC2 at specific serine/threonine sites, leading to exclusion from chromatin in mammalian cells. These phosphorylation events are reversed by protein phosphatase 1 (PP1) in late mitosis, allowing ORC reactivation in the subsequent G1.^[45][46][47] ORC regulation integrates with DNA damage checkpoints to maintain genomic stability. ATM and ATR kinases, activated by double-strand breaks or replication stress, phosphorylate ORC subunits such as ORC1 (at S196/S199), ORC3 (S208/S516), and ORC6 (T229), which stabilizes ORC on chromatin and facilitates recruitment of repair factors, thereby coordinating replication pausing with damage resolution. Additionally, CDKs shape the temporal program of origin firing by phosphorylating initiation factors downstream of ORC, ensuring early-firing origins are prioritized while dormant origins remain unlicensed until needed, as highlighted in recent analyses of replication dynamics.^[48][49] Experimental evidence from cell synchronization studies confirms these regulatory patterns. In synchronized human cells arrested in G1 (e.g., via serum starvation or thymidine block release), chromatin immunoprecipitation (ChIP) assays reveal peak ORC occupancy at replication origins during early G1, coinciding with maximal MCM loading, whereas occupancy diminishes in S/G2-arrested cells (e.g., via hydroxyurea) due to ORC1 export and core subunit modifications. Similar G1-specific peaks in ORC binding are observed in synchronized yeast cultures using alpha-factor arrest, underscoring the conserved temporal control of ORC activity.^[50]

Conservation Across Eukaryotes

The origin recognition complex (ORC) is a fundamental component of eukaryotic DNA replication initiation, present in all known eukaryotes from unicellular yeasts to multicellular humans, where it serves as the platform for loading the MCM helicase and licensing replication origins. Subunits Orc1 through Orc5 display high sequence conservation across these lineages, with amino acid identities ranging from 40% to 70% between budding yeast (Saccharomyces cerevisiae) and human homologs, reflecting their core structural and functional roles in ATP-dependent DNA binding and complex assembly. In contrast, Orc6 is the most divergent subunit, exhibiting low sequence similarity (often below 20%) and no structural homology to the other subunits, yet it remains essential for ORC integrity and pre-replicative complex (pre-RC) formation in diverse eukaryotes.^[5][6][22] Phylogenetic variations in ORC structure highlight adaptations to lineage-specific chromatin environments and replication needs. In metazoans, Orc1 contains a bromo-adjacent homology (BAH) domain at its N-terminus that binds histone H4 dimethylated at lysine 20 (H4K20me2), facilitating ORC recruitment to heterochromatic regions and stable chromatin association during development. Fungal Orc6, while nuclear in yeast, lacks the additional cytoplasmic and cytokinetic localizations seen in metazoan counterparts, emphasizing its primary role in replication rather than cell division. In protozoans like Trypanosoma brucei, ORC exhibits significant divergence, featuring Orc1/Cdc6, a divergent Orc4, and putative orthologs of Orc2 and Orc5, while orthologs of Orc3 and Orc6 are absent or highly modified, adapting to the parasite's polycistronic genome and unconventional replication control.^{[51][28][52][53]} Evolutionary studies underscore how sequence divergences enable functional flexibility while preserving ORC's essential role. A 2021 study demonstrated that "humanizing" yeast ORC by deleting a 19-amino-acid insertion helix in Orc4 abolishes sequence-specific binding to ARS consensus sequences, instead promoting stochastic, transcription start site-preferring interactions akin to human ORC, revealing this helix as a key evolutionary switch for origin selectivity in fungi. Complementarily, a 2025 analysis of CDK regulation in budding yeast showed mechanistic plasticity in MCM-ORC interactions, where Orc2's intrinsically disordered region enables loading at weak origins but is inhibited by CDK phosphorylation; this co-evolved with asymmetric origin architecture to prevent re-replication, implying broader eukaryotic adaptations in origin evolution driven by cell cycle constraints.^[22][54] Despite these variations, ORC maintains functional equivalence across eukaryotes in licensing replication origins for once-per-cell-cycle firing, highlighting a universal ATPase-dependent mechanism for pre-RC assembly that transcends sequence differences.^[22][6]

Pathological Implications

Associated Genetic Disorders

Meier-Gorlin syndrome (MGS) is an autosomal recessive primordial dwarfism disorder primarily caused by biallelic mutations in genes encoding components of the pre-replication complex, including ORC1, ORC4, ORC6, CDT1, and CDC6.^[55] These mutations disrupt the origin recognition complex's role in DNA replication initiation, leading to severe intrauterine and postnatal growth retardation, microcephaly, and bilateral microtia (underdeveloped ears).^[56] MGS represents the main monogenic disorder directly linked to ORC dysfunction, with cases often presenting additional features such as skeletal abnormalities and genitourinary malformations.^[55] Clinically, affected individuals exhibit profound short stature (often below the first percentile), delayed bone age, and characteristic facial dysmorphisms including a prominent forehead and microtia; intellectual disability is typically absent.^[55] Fewer than 150 cases of MGS have been reported worldwide as of 2024, with ongoing identification through genetic screening. At the molecular level, MGS-associated ORC mutations are predominantly hypomorphic, resulting in partial loss of function that impairs ORC assembly, chromatin binding, or MCM helicase loading without completely abolishing replication.^[57] For instance, the ORC1 R105Q missense mutation, identified in multiple MGS patients, disrupts the BAH domain's interaction with nucleosomal DNA and histone H4K20me2, thereby reducing ORC stability and replication origin licensing efficiency.^[57] Similar defects in ORC4 and ORC6 variants lead to tissue-specific reductions in pre-replicative complex formation, contributing to the syndrome's developmental phenotypes.^[58] These 2010s studies highlight how subtle perturbations in ORC function manifest as replication stress during rapid cell proliferation in embryonic tissues.^[59] Diagnosis of MGS relies on clinical evaluation combined with whole-exome or targeted sequencing to identify pathogenic variants in ORC or related genes, enabling early intervention for associated complications like recurrent infections.^[55] Therapeutic approaches remain supportive, focusing on growth hormone supplementation—which recent reviews indicate has shown variable efficacy in improving height in some patients—and monitoring for replication stress-related cellular vulnerabilities, though no targeted treatments exist as of 2025.^[55][60]

Involvement in Cancer

Dysregulation of the origin recognition complex (ORC) contributes to cancer progression by altering DNA replication licensing, leading to uncontrolled cell proliferation and genomic instability. Overexpression of ORC subunits, such as ORC6, has been documented in multiple tumor types, where it enhances oncogenic signaling and correlates with adverse clinical outcomes.^[61][62] In non-small cell lung cancer (NSCLC), ORC6 overexpression promotes tumor cell proliferation, migration, and invasion by facilitating excessive DNA replication initiation, as demonstrated in patient-derived tissues and cell line models. This upregulation is associated with advanced tumor stages and reduced overall survival, serving as a prognostic biomarker.^[61][63] Similarly, in glioma, elevated ORC6 expression drives growth and progression, correlating with higher tumor grades, wild-type IDH1 status, and poorer patient survival, based on analyses of human glioma samples and TCGA data.^[64][65] Pan-cancer analyses from The Cancer Genome Atlas (TCGA) reveal ORC family upregulation in most solid tumors, with ORC1 and ORC6 showing significant elevation across nearly all cancer types compared to normal tissues, occurring in a substantial proportion of cases. For instance, ORC1 amplification is observed in over 12% of hepatocellular carcinoma samples, contributing to aberrant replication.^[66][67][68] Aberrant ORC activity can induce re-replication, where licensed origins fire multiple times per cell cycle, resulting in replication stress and genomic instability that fuels oncogenesis. This is exemplified by ORC1 amplification in various cancers, which disrupts normal licensing controls and promotes DNA damage accumulation.^[69][70] Emerging evidence positions ORC as a therapeutic target, with inhibitors targeting ORC-ATPase activity under development to exploit replication vulnerabilities in cancer cells. Such approaches show promise in enhancing chemotherapy sensitivity and addressing replication stress in BRCA-deficient tumors, where ORC dysregulation exacerbates fork stalling.^[70][71] Recent studies highlight how motifs in ORC subunits regulate licensing efficiency, with disruptions linked to cancer-associated genomic alterations.^[72]