Previous clinical trials in breast cancer showed first evidence that CYP2D6 dose-adjusted treatment of tamoxifen resulted in significantly increased plasma concentrations of endoxifen, the major active metabolite of tamoxifen (Barginear et al., 2011; Irvin et al., 2011; Kiyotani et al., 2012; de Duenas et al., 2014; Dezentjé et al., 2015; Fox et al., 2016; Hertz et al., 2016). The gene discussed is CYP2D6; the disease is breast cancer.