Estrogen receptors (ERs) are a group of proteins found inside and on cells. They are receptors that are activated by the hormone estrogen (17ß-estradiol). Two classes of ER exist: nuclear estrogen receptors (ERa and ERß), which are members of the nuclear hormone family of intracellular receptors, and membrane estrogen receptors (mERs) (GPER (GPR30), ER-X, and G_q-mER), which are mostly G protein-coupled receptors. This article refers to the former (ER).