Smith-Lemli-Opitz syndrome (SLOS; McKusick 270400) is an autosomal recessive inherited metabolic-malformation disorder caused by deficient activity of 7-dehydrocholesterol reductase (DHCR7, E.C. 1.3.1.21), which catalyses the final step in the cholesterol-biosynthesis pathway. The clinical picture is characterized by a combination of congenital anomalies: microcephaly, hypotonia, incomplete development of the male genitalia, polydactyly, syndactyly of toes 2 and 3, cleft palate, heart and kidney malformations, failure to thrive and severe mental and growth retardation (Smith et al 1964). A decrease of plasma cholesterol and the accumulation of its precursor 7-dehydrocholesterol (7-DHC) is the biochemical hallmark in SLOS patients (Tint et al 1994). Cloning and sequncing of DHCR7 cDNA (Moebius et al 1998) and characterization of the human DHCR7 gene (Fitzky et al 1998) enabled investigation of defects of this gene at the DNA level. Several mutations have been described (Wassif et al 1998; Waterham et al 1998). Here we report the results of molecular analysis of the DHCR7 gene in 10 unrelated families with Smith-Lemli-Opitz syndrome. Results of mutation analyses are presented and compared with the clinical and biochemical data.