Steroid 11 beta-hydroxylase deficiency (11 beta OHD), an autosomal recessive hereditary disease, accounts for 5-8% of cases of congenital adrenal hyperplasia. In this study, we carried out a molecular genetic analysis of CYP11B1 encoding steroid 11 beta-hydroxylase (P450c11) from a Japanese patient affected with this disease. Nucleotide sequence analysis of polymerase chain reaction-amplified products of the patient's genome revealed the occurrence of a stop codon in exon 2 due to a point mutation, TGG-->TAG (Trp116-->Stop). To further analyze the role of CYP11B2 encoding steroid 18-hydroxylase (P450c18) in the 11 beta OHD patient, CYP11B2 of the patient was also amplified and sequenced. In contrast to CYP11B1, there was no mutation in CYP11B2. Restriction fragment length polymorphism analysis indicated that the 11 beta OHD patient is homozygous and his unaffected parents are heterozygous for the mutation. When a cDNA corresponding to CYP11B1 of the 11 beta OHD patient was transfected into COS-7 cells, steroid 11 beta-hydroxylase activity was not detectable in mitochondria of the cells. These results demonstrate that intact P450c11 was not produced at all due to the nonsense mutation in CYP11B1 of the patient without any mutation in CYP11B2.