NSDHL mutations in humans cause CHILD syndrome (congenital hemidysplasia with ichthyosiform erythroderma and limb defects), a rare X-linked dominant disorder with presumed lethality for CHILD causing alleles [34] and, with hypomorphic NSDHL mutations, CK syndrome, a form of X-linked mental retardation [22]. This evidence concerns the gene NSDHL and CK syndrome.