Smith-Lemli-Opitz syndrome: phenotypic extreme with minimal clinical findings.

Smith-Lemli-Opitz syndrome (SLO) is caused by inherited enzymatic deficiency of 7-dehydrocholesterol-delta7-reductase and resultant cholesterol deficiency. It comprises a characteristic combination of facial features, malformations, and mental retardation. We report on three related patients (two brothers and their first cousin) with mental retardation and minimal physical signs in whom the diagnosis of SLO was delayed for a number of years. The presence of a third-degree relative in the absence of consanguinity in this family supports the proposed high population carrier frequency. Our report suggests that cases of mild SLO remain undiagnosed and untreated, and that awareness of this common cause of mental retardation is low.