CYP2D6*88 is assigned uncertain function due to inadequate evidence. In vitro studies found CYP2D6*88 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). Therefore, consensus among experts was uncertain function.