CYP2D6*40 is assigned no function based on moderate evidence in heterozygous subjects and in vitro studies. CYP2D6*40 is defined by an 18 base pair insertion. CYP2D6*40 has been described in subjects with CYP2D6 *4/*40, CYP2D6 *6/*40, and CYP2D6 *40/*42 genotype demonstrating poor metabolism with a dextromethorphan substrate (12152006, 12811367). Multiple in vitro studies found CYP2D6*40 had nondetectable enzyme activity compared to wildtype with various substrates (24647041, 30366777). Therefore, consensus among experts was no function with an activity value of 0 based on moderate evidence.