CYP2D6*20 is assigned no function based on limited evidence. CYP2D6*20 was identified in a liver sample with decreased protein expression and decreased metoprolol metabolism (10471072). CYP2D6*20 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.