We analyzed the steroid sulfatase (STS) gene in nine Japanese patients with X-linked ichthyosis (XLI) by a polymerase chain reaction technique and subsequent DNA sequencing. Eight of nine patients showed complete deletion of the STS gene. In a patient of XLI exhibiting a normal amplifying pattern with predicted sizes of the STS gene, a novel mutation was found resulting in the appearance of a stop codon in exon 7 of the STS gene. This suggests that exon 7 or an area in its downstream region is important for STS activity.