Despite the large potential of MIPD, it is important to mention that the relationship between CYP2D6 genotype and/or endoxifen concentrations and breast cancer recurrence is still controversial and has been the subject of intensive debates amongst researchers and clinicians since almost a decade (Brauch et al., 2013; Binkhorst et al., 2015b; Zembutsu et al., 2017; Neven et al., 2018). The gene discussed is CYP2D6; the disease is breast carcinoma.