CYP2D6*4 is assigned no function based on strong evidence. CYP2D6*4 has consistently been described in subjects demonstrating decreased metabolism of various CYP2D6 substrates (2211621, 11266079, 1978251, 1978565). Two in vitro studies found CYP2D6*4 had undetectable protein expression (2211621, 11266079). CYP2D6*4 is defined by a splicing defect resulting in a nonfunctional protein. Therefore, consensus among experts was no function with an activity value of 0 based on strong evidence.