 The human cytochrome P450 CYP superfamily comprises 57 genes. These genes code for a myriad of enzymatic reactions. The cytochrome P450 CYP2C9 is responsible for metabolism of the S enantiomer of warfarin. Two allelic variants, CYP2C92 and CYP2C93, differ from the wild type CYP2C91 by a single amino acid substitution. The allelic variants are associated with impaired hydroxylation of S-warfarin. 