CYP2D6*4x2 is assigned no function based on moderate evidence. CYP2D6*4x2 has been described in subjects demonstrating decreased metabolism of dextromethorphan or debrisoquine (10022755, 12152006). CYP2D6*4 is defined by a splicing defect resulting in a nonfunctional protein and gene duplication would therefore not impact enzyme activity. Therefore, consensus among experts was no function with an activity value of 0 based on moderate evidence.