AP1S1 mutations cause MEDNIK (mental retardation, enteropathy, deafness, peripheral neuropathy, ichthyosis, and keratoderma) syndrome [21]; AP1S2 mutations are responsible for some forms of X-linked mental retardation [18]; and AP1S3 mutations increase susceptibility to pustular psoriasis [22,39]. The gene discussed is AP1S3; the disease is ichthyosis.