ESR1 and breast cancer: Recently, we reported that ER-positive breast cancer cells expressing high levels of ER-α36 are more resistant to antiestrogen tamoxifen [27], consistent with our previous report that the breast cancer patients with tumors expressing high levels of ER-α36 less benefited from tamoxifen therapy compared to those with low levels of ER-α36 expression, and ER-α36 expression is significantly associated with HER2 expression [22], suggesting that increased ER-α36 expression is one of the underlying mechanisms of tamoxifen resistance.