CYP2D6 and breast cancer: CYP2D6 phenotype related alteration of endoxifen plasma levels in tamoxifen treated postmenopausal early breast cancer women is well established and contributes to treatment outcome (Borges et al. 2006; Kiyotani et al. 2010; Mürdter et al. 2011; Lim et al. 2011; Madlensky et al. 2011).