CYP2C9*4 is assigned decreased function based on limited evidence in heterozygous patients and in vitro studies. One patient on a stable dose of warfarin with CYP2C9*1/*4 genotype required a lower weekly warfarin dose compared to CYP2C9*1/*1 patients (21228733). In an in vitro study CYP2C9*4 demonstrated 16% enzyme activity with a warfarin substrate and 22% enzyme activity with a tolbutamide substrate compared to wildtype. The CYP2C9*4 defining SNP occurs in the substrate recognition site which likely contributes to the loss of enzyme activity (23752738). 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.