CYP2D6*99 is assigned no function based on limited evidence. CYP2D6*99 has been described in a subject with CYP2D6*5/*99 genotype demonstrating markedly decreased metabolism compared to wildtype with dextromethorphan and venlafaxine substrates (29327975). CYP2D6*99 is defined by the same SNPs as CYP2D6*10 in addition to a SNP resulting in an amino acid substitution which disrupts enzyme activity yielding a nonfunctional protein. Therefore, consensus among experts was no function with an activity value of 0 based on limited evidence.