Patients with this genotype have reduced metabolism of metoprolol.
References
Bijl MJ, Visser LE, van Schaik RH, Kors JA, Witteman JC, Hofman A, Vulto AG, van Gelder T, Stricker BH: Genetic variation in the CYP2D6 gene is associated with a lower heart rate and blood pressure in beta-blocker users. Clin Pharmacol Ther. 2009 Jan;85(1):45-50. doi: 10.1038/clpt.2008.172. Epub 2008 Sep 10. [Article]