CYP2C9*31 is assigned decreased function based on limited evidence. Multiple in vitro studies using different cell systems found CYP2C9*31 demonstrated decreased clearance for numerous substrates compared to wildtype (23752738, 24118918, 23844998, 25144335, 25951663, 25476996, 25832633, 25884291, 27163851, 25924705, 27199745, 24077631). Although one in vitro study showed CYP2C9*31 had similar activity compared to wildtype, the experts did not find this result convincing as in this study CYP2C9*2 and CYP2C9*3 which definitively have decreased function and no function, respectively, demonstrated increased activity compared to wildtype (25142737). Therefore, consensus among experts was decreased function with an activity value of 0.5 based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function alleles.