STS and recessive X-linked ichthyosis: Approximately 90% of cases of XLI are caused by deletions within, or encompassing, the steroid sulfatase (STS) gene on chromosome Xp22.31, with ~ 10% caused by point mutations within the STS gene [3]; deletions may be “typical” (encompassing STS and a small number of adjacent genes), or “atypical” (encompassing many contiguous genes).