In liver samples from 19 Caucasian subjects, the CYP3A5 protein was detected in 74% of individuals (14/19), while the messenger was shown to be expressed in 100% of individuals, assessed by the RT-PCR method. In order to characterise the putative mutation(s) in the messenger, accounting for the absence of protein accumulation, the full coding region of the CYP3A5 cDNA was sequenced for two unrelated individuals, one expressing the protein at a high level and one being defective. A point mutation in exon 11 at position 1280 (C-->A) was found to cosegregate with the absence of protein accumulation in 2 of the 5 defective individuals. This mutation produces a change in the amino acid at position 398 (Thr-->Asn). Whether this mutation affects the stability of the protein or whether it is in linkage desequilibrium with other mutation(s) in the non-coding region of the messenger is not known. The C-->A change at 1280 generates a Tsp509 I site which can be used for routine evaluation of the frequency of this mutation in population studies.