ESR1 and breast cancer: Tamoxifen is an anti-estrogen that selectively blocks estrogen binding to ERα through competitive inhibition.25–27 Though tamoxifen is mainly used in adjuvant therapy of ERα-positive breast cancers, some clinical studies have shown that this drug was effective in some ERα-negative tumors.28 ERβ has been shown to be a predictive marker for favorable prognosis in response to adjuvant tamoxifen therapy of ERα-negative tumors including TNBC.3,29–32 However, none of these studies have taken into consideration the status of p53 in the tumors.