STS and syndromic recessive X-linked ichthyosis: According to the International Classification of Corneal Dystrophies (IC3D), PDCD is classified into two subtypes: (1) isolated PDCD, with unknown genetic locus; (2) PDCD associated with X-linked ichthyosis, a deletion of steroid sulfatase (STS) gene on chromosome Xp22.3 (MIM #308100) [3].