Cerebrotendinous xanthomatosis (CTX) is an autosomal recessive sterol storage disease characterized by the accumulation of a bile alcohol, cholestanol, in diverse tissues. The disorder is manifested by extensive nervous system involvement, juvenile cataracts, tendon xanthomas, and premature atherosclerosis and is caused by sterol 27-hydroxylase (EC 1.14.13.15) mutations. Recently, two mutations were shown to cause CTX in four Jewish families of Moroccan origin. An additional mutant allele, found in a Jewish family of Algerian origin is characterized here. Sequence analysis revealed a C to T transition at cDNA position 1037 which predicted a threonine to methionine substitution at residue 306 (designated T306M). It is highly suggestive, but not definitive, that this transition is the mutation causing CTX in this family. A search for additional cases from Jewish families of North African extraction identified five new families including 10 cases. The three sterol 27-hydroxylase gene mutations account for all 10 CTX families and their presence may suggest the existence of positive selective forces that lead to an increased prevalence of this relatively rare disease in Jews from North Africa.