Cytochrome P450 3A4 (CYP3A4) and CYP3A5 are the main enzyme responsible for metabolism of tacrolimus. Unlike CYP3A4, CYP3A5 is more polymorphic and expression of enzyme is controlled by CYP3A5 gene variants. CYP3A5*3, an A to G transition (A6986G) within intron 3 results in the production of a truncated protein, is the most common allele variant. From previous study of our group, the allele frequency of CYP3A5*3 was 66% which is lower than those reported in the Caucasian population (85%). Prevalence of CYP3A5*1/*1, CYP3A5*1/*3, CYP3A5*3/*3 in 400 healthy Thai volunteers are 13.75%, 42.75% and 43.50%, respectively (unpublished data).