ESR1 and breast cancer: Of the two different types of receptors, namely, the estrogen receptor-α (ERα) and -β (ERβ) [24], activation of ERα signaling leads to the proliferation of breast cancer cells via induction of proliferating cell nuclear antigen (PCNA), and suppression of p53 and p21 [25], whereas ERβ signaling inhibits the growth of breast cancer cells [26, 27].