X-linked ichthyosis (abbreviated XLI) is a skin condition caused by the hereditary deficiency of the steroid sulfatase (STS) enzyme that affects 1 in 2000 to 1 in 6000 males. XLI manifests with dry, scaly skin and is due to deletions or mutations in the STS gene. XLI can also occur in the context of larger deletions causing contiguous gene syndromes. Treatment is largely aimed at alleviating the skin symptoms. The term is from the Ancient Greek 'ichthys' meaning 'fish'. Signs and symptoms The major symptoms of XLI include scaling of the skin, particularly on the neck, trunk, and lower extremities. The extensor surfaces are typically the most severely affected areas. The >4 mm diameter scales adhere to the underlying skin and can be dark brown or gray in color. Symptoms may subside during the summer. Associated medical conditions Aside from the skin scaling, XLI is not typically associated with other major medical problems. Atrial fibrillation or atrial flutter may affect up to 1 in 10 males with XLI. Heart rhythm abnormalities in individuals with XLI tend to co-occur with disorders of the gastrointestinal tract, and are likely to result from steroid sulfatase deficiency. Corneal opacities may be present but do not affect vision. Cryptorchidism is reported in some individuals. Individuals with XLI appear at increased risk of developmental disorders such as autism and Attention Deficit Hyperactivity Disorder and some affected individuals exhibit mood problems Mood problems in XLI appear to be most influenced by stigma or bullying associated with the skin condition, and by difficulties with treating the skin condition. Individuals with XLI can exhibit intellectual disability, although this is thought to be due to deletions encompassing neighboring genes (e.g. VCX) in addition to STS. The skin and medical conditions associated with XLI are likely to be due to perturbed basement membrane function and abnormal interactions with the extracellular matrix Female carriers generally do not experience any of these problems but can have difficulty during childbirth, as the STS expressed in the placenta plays a role in normal labor. Female carriers may also be at slightly increased risk of developing mental health problems following childbirth For these reasons carriers should ensure their obstetrician is aware of the condition. Genetics The STS gene is located on the X chromosome at band Xp22.3. Thus, the syndrome is an X-linked condition, and it affects males and females differently. The 23rd pair of chromosomes