CYP2D6*31 is assigned no function based on moderate evidence in heterozygous subjects and in vitro studies. CYP2D6*31 was first identified in a subject with CYP2D6 *4/*31 genotype demonstrating markedly decreased metoprolol metabolism with an AUC more than three times greater than the AUC of wildtype subjects and similar AUC to subjects with two known nonfunctional alleles (15726636). CYP2D6*31 was described in another subject demonstrating decreased dextromethorphan metabolism based on urine metabolites (20473659). Multiple in vitro studies found CYP2D6*31 had nondetectable or markedly decreased enzyme activity compared to wildtype with various substrates (15726636, 24647041, 30366777). CYP2D6*31 is defined by three amino acid substitutions and an analysis of the crystal structure and docking study suggested the Arg440His substitution causes reduced reductase binding resulting in impaired enzyme activity (15726636). Therefore, consensus among experts was no function with an activity value of 0 based on moderate evidence.