CYP2C9*18 is assigned uncertain function due to inadequate evidence. In one in vitro study, CYP2C9*18 protein expression could not be detected in an E.coli system (16099926). CYP2C9*18 protein expression was detected in a separate study using COS-7 cells and no enzyme activity was detected with warfarin and tolbutamide substrates (23752738). Therefore, consensus among experts was uncertain function due to insufficient evidence.