In a recent matched case–control study in 3,901 breast cancer survivors (Goetz et al. 2013), patients were treated over 5 years either with tamoxifen alone, or after initial treatment with tamoxifen for 2 years, were switched to anastrozole, an aromatase inhibitor, which is neither a pro-drug nor metabolised by cytochrome CYP2D6. The gene discussed is CYP19A1; the disease is breast cancer.