STS and Hyperkeratosis: Most cases of XLI are caused by an X-linked genetic mutation resulting in deficiency for the enzyme steroid sulfatase (STS), an enzyme which cleaves sulfate groups from a range of steroids; the accumulation of cholesterol sulfate in the stratum corneum as a consequence of STS deficiency in XLI results in increased intracellular stability and cohesion, and, ultimately, partial retention hyperkeratosis and scaling [1].