Previous studies have demonstrated that estrogens contribute to the development of breast cancer mainly acting through the classical estrogen receptors (ERs) [10]; however, the G protein estrogen receptor (GPER) has been also involved in the estrogen stimulation of both ER-negative breast cancer cells and CAFs [11,12,13,14,15,16,17,18,19,20,21]. Here, ESR1 is linked to breast cancer.