ESR1 and ovarian cancer: More than 80% of high-grade serous or endometrioid carcinomas of the ovary express hormone receptors, in particular, the estrogen receptor (ER) Erα.8,9 Experimental data show the activity of estrogen on ovarian cancer cell proliferation in cell culture and in a xenograft model.9,10 The activity of aromatase inhibitors on ovarian cancer cells expressing ERα has been demonstrated in a mouse model.11