Specifically, CYP2D6 contributes to hepatic metabolism of ~25% of drugs in clinical use, including many antidepressants, antipsychotics, opioids, antiemetics, anti-arrhythmics, β-blockers, cancer chemotherapeutics and drugs of abuse.1,2 The enzymatic activity of CYP2D6 varies widely among individuals, based both on level of expression and on functional genetic variations (alleles), resulting in significant clinical consequences for drug metabolism and individual risk of adverse events or drug efficacy (www.cypalleles.ki.se/3 and www.pharmgkb.org/). The gene discussed is CYP2D6; the disease is cancer.