The immunosuppressant mycophenolate mofetil (MMF) is metabolized by several uridine diphosphate glucuronosyltransferases (UGTs), among which UGT1A9 is the most important enzyme. Two commonly occurring single nucleotide polymorphisms (SNPs) in the promoter region of the UGT1A9 gene have been shown to enhance UGT1A9 protein level and activity in vitro. This study is to investigate the potential in vivo effects of these two SNPs on the metabolism and hence the pharmacokinetics of MMF metabolites. One hundred and thirty healthy volunteers will be screened for the presence of UGT1A9 C-2152T and T-275A polymorphisms. Seventeen subjects who carry the variant alleles, along with 17 others who do not have the polymorphisms, will be selected to participate in the pharmacokinetic study. Pharmacokinetics of the metabolites mycophenolic acid (active) and mycophenolate 7-O-phenolic glucuronide (inactive) will be determined, and parameter values will be compared between the two groups. It is hypothesized that the metabolism of MMF is enhanced in subjects with the two SNPs, resulting in an increase in clearance and a decrease in the exposure of mycophenolic acid.