X-linked ichthyosis (XLI, OMIM:308100), which typically results in the early onset of dark, dry, and irregular scales affecting the limb and trunk, is caused by the deletion of the STS gene, encoding steroid sulfatase and located near ANOS1 at Xp22.3 (Takeichi and Akiyama, 2016). The gene discussed is STS; the disease is recessive X-linked ichthyosis.