What genetic defect is responsible for X-linked ichthyosis (XLI)?

Ninety percent of patients with XLI have deletions in the STS gene on chromosome Xp22.3, leading to steroid sulfatase deficiency and the classic phenotype of XLI. However, some patients have larger deletions at this site, encompassing neighboring genes. These patients present with more complicated forms of XLI representing contiguous gene deletion syndromes (OMIM [Online Mendelian Inheritance in Man] #308100).