The cytochrome P450 enzymes CYP2C8 and CYP2C9 play important roles in drug metabolism (1)(2)(3). Single-nucleotide polymorphisms that have been identified in the genes encoding for these enzymes may contribute to variability in drug response through alterations in metabolism. CYP2C8 expression includes five alleles: CYP2C8*1 (wild type), CYP2C8*2 , CYP2C8*3 (1), CYP2C8*4 (2), and CYP2C8*5 (4). The variant alleles CYP2C8*2 and * 3 have been shown to encode for an enzyme with decreased activity in vitro (1), whereas CYP2C8*4 expression appears to cause decreased CYP2C8 activity (2). The allele frequency of CYP2C8*2 is 0.18 in the African-American population, whereas CYP2C8*3 and CYP2C8*4 are expressed primarily in the Caucasian population at frequencies of 0.13 and 0.075, respectively (1)(2). Thirteen CYP2C9 alleles have been identified to date, and the majority of studies on these have focused on the importance of CYP2C9*2 (5) and CYP2C9*3 (6). CYP2C9*4 is expressed in the Asian population at an allele frequency of 0.02, but does not appear to be expressed in Caucasians or African Americans (7). CYP2C9*5 and * 6 are found primarily in African Americans at frequencies of 0.017 (8) and 0.006 (9), respectively. Blaisdell et al.(10) recently identified variants designated as CYP2C9*7 through * 12 , which have allele frequencies ranging from 0.006 to 0.03 in African populations. The CYP2C9*8 , * 11 , and * 12 alleles have altered in vitro metabolic activities compared with the wild-type allele, CYP2C9*1 (10). Finally, CYP2C9*13 was identified in a Chinese population with an allele frequency of 0.01 and was associated with decreased CYP2C9 activity (11). Given the importance of these enzymes in the metabolism of multiple drugs, a reliable genotyping method …