ESR1 and breast cancer: Importantly, while HT is a conventional approach for relieving postmenopausal syndrome, the long-term use of estrogen in HT imparts an increased risk of breast cancer (Colditz, 1998), and estrogen can be mediated by estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) (Deroo and Korach, 2006; Jia et al., 2015).