ESR1 and breast cancer: Since the first evidence that ER-β is an important modulator of proliferation and invasion of BC cells, it has been shown that the ratio of ER-α/ER-β expression is higher in BC than normal tissues due to the lower expression of ER-β, supporting the hypothesis first shown by League et al[47] that the loss of ER-β expression could be one of the events leading to the development of BC tumorigenesis.