CYP2D6*12 is assigned no function based on limited evidence. CYP2D6*12 is defined by a nonsense mutation yielding a nonfunctional enzyme and was identified in a subject demonstrating markedly decreased sparteine metabolism (8655150, 9241659). Two in vitro studies found CYP2D6*12 had nondetectable enzyme activity with various substrates (24647041, 30366777). 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.