ESR1 and breast cancer: Further studies have supported the hypothesis of a estrogen-mediated membrane bound G protein-coupled receptor; in one such study demonstrating that 17β-estradiol is successful in activating ERK-1 and ERK-2 and adenylyl cyclase in cells lines expressing ERα and ERβ (MCF-7) and also in breast cancer cell lines (SKBR3) which do not express either receptor but do express GPR30 [107].