Cunniff et al. (1997) reported the clinical and biochemical spectra of 80 patients (68 index cases and 12 family members) with abnormally increased levels of 7-dehydrocholesterol. The phenotypic spectrum ranged from isolated syndactyly of toes 2 and 3 to holoprosencephaly and multiple visceral anomalies resulting in death in utero. Plasma cholesterol concentration was inversely correlated with clinical severity. Little relationship was seen between severity score and 7-dehydrocholesterol concentration. However, 10% of patients had normal serum cholesterol concentrations and would have been missed without quantification of 7-dehydrocholesterol. Syndactyly of toes 2 and 3 was found in 79 of the 80 patients. Johnson (1975) reported 2/3 toe syndactyly in only 73% of his 55 SLO syndrome patients. This finding suggested to Cunniff et al. (1997) that as many as one quarter of previously documented SLOS patients may have had a different genetic disorder.