CYP2D6*21 is assigned no function based on limited evidence. CYP2D6*21 has been described in subjects demonstrating markedly decreased metabolism of debrisoquine and dextromethorphan (10471060, 15618746). CYP2D6*21 is defined by a base insertion resulting in a frameshift and premature stop codon, yielding a truncated and nonfunctional enzyme. Therefore, consensus among experts was no function with an activity value of 0 based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal and decreased function alleles.