Patients with this genotype have reduced metabolism of nortriptyline.
References
Bijl MJ, Visser LE, Hofman A, Vulto AG, van Gelder T, Stricker BH, van Schaik RH: Influence of the CYP2D6*4 polymorphism on dose, switching and discontinuation of antidepressants. Br J Clin Pharmacol. 2008 Apr;65(4):558-64. Epub 2007 Dec 7. [Article]