The members of the cytochrome P450 (CYP) 3A subfamily play an important role in the metabolism of more than 50% of the drugs metabolized by CYPs. Among the CYP3A members, CYP3A5 is known to exhibit polymorphic expression within the human liver. We hypothesized that a single nucleotide polymorphism (SNP) in the 5'-regulatory region of the CYP3A5 gene might be the cause of CYP3A5 polymorphic expression. Due to the existence of "CYP3AP1," a highly homologous sequence to the CYP3A5 gene, it was necessary to make specific primers to the CYP3A5 gene. In the present study, we designed a series of oligonucleotide primers for sequencing the proximal promoter region of the CYP3A5 gene in order to search for the putative regulatory single nucleotide polymorphism. We examined 86 established cell lines derived from Japanese individuals as a representation of the Japanese population. However, no SNP was detected in the promoter region of the CYP3A5 gene isolated from the cell lines used, suggesting other causal factors for the observed polymorphism of CYP3A5-dependent drug metabolism.