Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive disorder believed to be exclusively caused by mutations in the CYP27A1 gene coding for the enzyme sterol 27-hydroxylase. Common findings in CTX are tendon xanthomas, cataracts and progressive neurological dysfunction. Here, we characterize an adult female patient with tendon xanthomas and classic biochemical findings of CTX (i.e. high levels of bile alcohols and cholestanol and extremely low levels of 27-hydroxycholesterol in plasma). Additionally, sterol 27-hydroxylase activity in cultured monocyte-derived macrophages from this patient was <5% of normal. Sequencing the CYP27A1 gene uncovered that the patient is heterozygous for two previously undescribed base substitutions in exon 8, C478A and C479A, which are expected to affect the haeme-binding domain of the enzyme. When expressed in HEK293 cells, the corresponding protein had only 8% of normal enzymatic activity. No other mutation was found in the open reading frame of the CYP27A1 gene, intron-exon boundaries or in the 5'-untranslated region up to 5000 bp distal to the translational start site. Sequencing mRNA isolated from leucocytes from the patient revealed a 1 : 1 ratio of mutated and nonmutated species, with total mRNA levels that were not significantly different from the controls. It is concluded that the patient is heterozygous for two mutations affecting one allele of the CYP27A1 gene and with at least one additional yet undefined gene that is of critical importance for the activity of sterol 27-hydroxylase.