CYP2D6*91 is assigned decreased function with an activity value of 0.25 due to limited evidence. *91 contains 2989G>A splicing SNP found in *41 (decreased function). In vitro studies found CYP2D6*91 demonstrated enzyme activity ranging from similar to decreased activity compared to wildtype with various substrates (25469868, 26310775, 26666748, 26544071, 26406933, 27109434, 27097346, 27016952, 26542054, 26937172, 26968424, 27203132, 27251229, 27354764, 27538917): however, in the cases of normal activity, the effect of the 2989G>A splice SNP found in *41 was not captured in the in vitro system utilized which may explain activity comparable to *1 or *2. Therefore, consensus among experts was decreased function based on limited evidence.