CYP2D6*9 is assigned decreased function based on moderate evidence. CYP2D6*9 was first identified in a liver sample with decreased protein expression and enzyme activity (1844820). Subjects with CYP2D6*9 demonstrated decreased debrisoquine metabolism but greater metabolism compared to subjects with no function alleles based on urine metabolites, indicating there is not a complete loss of enzyme function (8101460). Similarly, liver microsomes of two subjects with CYP2D6*4/*9 and CYP2D6 *1/*9 genotype demonstrated decreased intrinsic clearance compared to wildtype with a bufuralol substrate (11266079). Two in vitro studies found CYP2D6*9 had a modest decrease in intrinsic clearance compared to wildtype (24647041, 30366777). Although one in vitro experiment found CYP2D6*9 had similar activity compared to wildtype with various substrates, the result was not convincing as decreased protein expression in vivo likely contributes to decreased enzyme activity observed in subjects carrying the CYP2D6*9 allele (1844820). Pharmacogenomic clinical studies found the measured activity values for several substrates is reduced (0.34 (PMID 30312494), 0.18 (PMID 33792048), 0.28 (PMID 33792048), 0.22 (PMID 32599653); 0.46 PMID 33932135) and a lower activity value of 0.25 is warranted for most substrates. Therefore, consensus among experts was decreased function with an activity value of 0.25 based on moderate evidence.