CYP2D6*128 is assigned uncertain function due to inadequate literature describing function of this allele. One in vitro study demonstrated substantially decreased intrinsic clearance CLint (Vmax/Km) of 3.3% compared to *1 for tamoxifen N-desmethylation (32759992); no activity was detectable for dextromethorphan O-demethylation. Therefore, consensus among experts was uncertain function due to inadequate evidence.