ESR1 and breast cancer: Our previous study showed that β2M small interfering RNA (siRNA) could significantly inhibit the B-cell lymphoma 2 (Bcl-2) expression, but not the estrogen receptor (ER), progesterone receptor (PR), and HER2 expression in breast cancer cells MCF-7 with ER-positive (ER+), PR-positive (PR+), and HER2-negative (HER2−) status, whereas the β2M siRNA significantly upregulated the Bcl-2 and HER2 expression in breast cancer cells MDA-MB-231 with ER-negative (ER−), PR-negative (PR−), and HER2− status [3].