ESR1 and breast cancer: The activity, selectivity, and GPER dependence of G-1 have been demonstrated in a number of ER-negative cell lines, including SKBR3 (breast cancer) (19), Hec50 (endometrial cancer) (83), and MCF10A (normal breast epithelium) (84) cells, employing small interfering RNA knockdown approaches (32, 83, 84) as well as in multiple systems in GPER knockout (KO) mice (85).