The activation of GPR30 can stimulate adenylyl cyclase, transactivate epidermal growth factor receptors (EGFRs), induce mobilization of intracellular calcium (Ca2+) stores, and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) signaling pathways.6,7 Previous studies revealed that GPR30 can modulate growth of hormonally responsive cancers such as endometrial,8 ovarian,9 and breast cancer.10 Therefore, GPR30 likely has an important role in modulating estrogen responsiveness and development and/or progression of ER− breast cancer. This evidence concerns the gene GPER1 and breast cancer.