CYP2C9*9 is assigned normal function based on strong evidence in heterozygous patients, homozygous patients, and in vitro studies. Twenty-one subjects with CYP2C9 *1/*9 genotype and three subjects with CYP2C9 *9/*9 genotype had similar urinary excretion of a phenytoin metabolite compared to CYP2C9 *1/*1 subjects following a phenytoin 300mg dose, demonstrating CYP2C9*9 is not associated with decreased phenytoin metabolism (16220110). Seven subjects with CYP2C9 *1/*9 genotype had similar acenocoumarol plasma concentrations 8 and 24 hours after an 8mg dose compared to CYP2C9 *1/*1 subjects (21811894). Seven patients on a stable dose of warfarin with CYP2C9 *1/*9 genotype had a similar maintenance warfarin dose compared to CYP2C9 *1/*1 patients (20716240). In vitro studies demonstrated CYP2C9*9 intrinsic clearance or enzyme activity was similar compared to wildtype using warfarin and tolbutamide substrates (23752738, 15284535). Although one in vitro study showed CYP2C9*9 demonstrated decreased enzyme activity with a 7-ethoxycoumarin substrate compared to wildtype (28758225), the experts did not find this result convincing as no clinical difference was seen in patients taking acenocoumarol or warfarin, or in another in vitro study using a warfarin substrate as previously described. Although one study using the polyphen computational program to predict the functional effect of the SNP defining CYP2C9*9 predicted the substitution as possibly damaging to enzyme function, the authors also noted that this effect has not been shown in clinical phenotypic studies (19164093). Therefore, consensus among experts was normal function with an activity value of 1 based on strong evidence.