Cerebrotendinous xanthomatosis (CTX) is an inherited sterol storage disease associated with the accumulation of cholestanol and cholesterol in various tissues. CTX is caused by a deficiency of sterol-27-hydroxylase, a mitochondrial enzyme that oxidizes the side chain of cholesterol in the pathway leading to the formation of bile acids. In the present study we report two mutations of sterol-27-hydroxylase gene (CYP27 gene) found in Italian CTX patients. Proband T.C. is homozygous for a G-->A transition at the first nucleotide of intron 7. This mutation causes the formation of minute amounts of an abnormal mRNA, in which exon 6 joins directly to exon 8 with the skipping of exon 7. The exon 6-exon 8 junction results in a frame shift, downstream from the codon for Arg362, which generates a string of 28 novel amino acids preceding a premature termination codon. Proband C.U. is homozygous for a G-->C transversion at the last nucleotide of exon 3. This mutation, which changes the consensus sequence of the 5' donor splice site, is associated with barely detectable levels of sterol-27-hydroxylase mRNA, of normal size, in proband fibroblasts. As both mutations change the sites for two restriction enzymes, rapid methods were devised for the identification of the healthy carriers among the probands' family members and for the screening of these mutations in other CTX patients.