CYP2C9*13 is assigned no function based on definitive evidence in heterozygous patients. Subjects with CYP2C9*1/*13 genotype repeatedly demonstrated decreased clearance and increased AUC compared to wildtype subjects and similar clearance and AUC compared to CYP2C9*1/*3 subjects in multiple studies with different substrates (19604036, 21395648, 21726410, 21842338, 22735459, 27377818, 27864660, 15606435). Two studies correlated in vitro data with clinical data and also found increased AUC and decreased clearance compared to wildtype with lornoxicam and tolbutamide (15764711, 25614704). Additionally, a CYP2C9*3/*13 subject had a greater decrease in tolbutamide clearance compared to the CYP2C9*1/*13 subject (25614704). This was also demonstrated in another CYP2C9*3/*13 subject with decreased lornoxicam clearance and increased AUC compared to wildtype, CYP2C9 *1/*3, and CYP2C9*1/*2 subjects (15606435). Multiple in vitro studies demonstrate CYP2C9*13 exhibits decreased metabolism comparable to that observed for CYP2C9*3 for numerous substrates (23844998, 24118918, 24464553, 25142737, 25476996, 25832633, 25884291, 27163851, 24077631, 25951663, 19541829, 20814155, 23752738, 16308280, 25144335) and one model suggests the decreased metabolism across substrates is due to a side chain rotation which blocks the catalytic site (16740353). Therefore, consensus among experts was no function with an activity value of 0 based on definitive evidence.