Although the presence of ERα is a well-established predictor of response to TAM treatment in BC (Droog et al., 2013; Rondón-Lagos et al., 2016), evidence also exists that some ERα-negative tumors benefit from adjuvant TAM treatment (Gruvberger-Saal et al., 2007; Manna and Holz, 2016; Rondón-Lagos et al., 2016). This evidence concerns the gene ESR1 and breast cancer.