CYP2D6*133 is assigned uncertain function due to inadequate literature describing function of this allele. One in vitro study demonstrated substabtially decreased intrinsic clearance CLint (Vmax/Km) of 2.5% and 4.5% compared to *1 for tamoxifen N-desmethylation and dextromethorphan O-demethylation, respectively. Therefore, consensus among experts was uncertain function due to inadequate evidence.