Harlequin-type ichthyosis is a genetic disorder that results in thickened skin over nearly the entire body at birth. The skin forms large, diamond/trapezoid/rectangle-shaped plates that are separated by deep cracks. These affect the shape of the eyelids, nose, mouth, and ears and limit movement of the arms and legs. Restricted chest movement can lead to breathing difficulties. These plates fall off over several weeks. Other complications can include premature birth, infection, problems with body temperature, and dehydration. The condition is the most severe form of ichthyosis (except for syndromes that include ichthyosis, for example, Neu–Laxova syndrome), a group of genetic disorders characterised by scaly skin.

Harlequin-type ichthyosis is caused by mutations in the ABCA12 gene. This gene codes for a protein necessary for transporting lipids out of cells in the outermost layer of skin. The disorder is autosomal recessive and inherited from parents who are carriers. Diagnosis is often based on appearance at birth and confirmed by genetic testing. Before birth, amniocentesis or ultrasound may support the diagnosis.

There is no cure for the condition. Early in life, constant supportive care is typically required. Treatments may include moisturizing cream, antibiotics, etretinate or retinoids. Around half of those affected die within the first few months; however, retinoid treatment can increase chances of survival. Children who survive the first year of life often have long-term problems such as red skin, joint contractures and delayed growth. The condition affects around 1 in 300,000 births. It was first documented in a diary entry by Reverend Oliver Hart in America in 1750.

Newborns with harlequin-type ichthyosis present with thick, fissured armor-plate hyperkeratosis. Sufferers feature severe cranial and facial deformities. The ears may be very poorly developed or absent, as may the nose. The eyelids may be everted (ectropion), which leaves the eyes and the area around them very susceptible to infection. Babies with this condition often bleed during birth. The lips are pulled back by the dry skin (eclabium).

Joints sometimes lack movement and may be below the normal size. Hypoplasia is sometimes found in the fingers. Polydactyly has been found on occasion. The fish mouth appearance, mouth breathing, and xerostomia place affected individuals at extremely high risk for developing rampant dental decay.

People with this condition are extremely sensitive to changes in temperature due to their hard, cracked skin, which prevents normal heat loss. The skin also restricts respiration, which impedes the chest wall from expanding and drawing in enough air. This can lead to hypoventilation and respiratory failure. Patients are often dehydrated, as their plated skin is not well suited to retaining water.

Harlequin-type ichthyosis is caused by a loss-of-function mutation in the ABCA12 gene. This gene is important in regulating protein synthesis for the development of the skin layer. Mutations in the gene cause impaired transport of lipids in the skin layer and may also lead to shrunken versions of the proteins responsible for skin development. Less severe mutations result in a collodion membrane and congenital ichthyosiform erythroderma-like presentation.

ABCA12 is an ATP-binding cassette transporter (ABC transporter), which is a member of a large family of proteins that hydrolyze ATP to transport cargo across cell membranes. ABCA12 is thought to be a lipid transporter in keratinocytes necessary for lipid transport into lamellar granules during the formation of the lipid barrier in the skin.