The best known is Smith-Lemli-Opitz Syndrome, in which patients have central nervous system (CNS) malformations, including holoprosencephaly and microcephaly, and skeletal defects (most often postaxial polydactyly) caused by mutations in 7-dehydrocholesterol reductase (DHCR7) [15]. The gene discussed is DHCR7; the disease is microcephaly.