CYP2D6*28x2 was deposit into PharmVar. CYP2D6*28 is assigned uncertain function due to inadequate evidence. In vitro studies found CYP2D6*28 demonstrated enzyme activity ranging from nondetectable to similar activity compared to wildtype with various substrates (24647041, 30366777). Therefore, consensus among experts was uncertain function for CYP2D6*28. CYP2D6*28x2 is also assigned uncertain function as it cannot yet be determined what the phenotype of a duplicated allele with uncertain function would be clinically similar to.