ESR1 and breast cancer: Upregulation of HER2 in the tamoxifen-resistant, ER+, MCF7 sublines has previously been reported in different studies, and upregulation of this hormone receptor is also known to promote tamoxifen/endocrine resistance in ER+ breast cancer (Benz et al., 1992; Arpino et al., 2004; Huang et al., 2017).