ESR1 and breast carcinoma: In order to test the activity of BU-32, irrespective of estrogen receptor status, we used a panel of breast cancer cell lines: MCF-7 (estrogen receptor-positive, progesterone receptor-positive), MDA-MB-231 (estrogen receptor-negative, progesterone receptor-negative, HER2-negative) and SKBR3 (HER-positive).