Although originally identified as an orphan receptor (under the name GPR30), a study in SK-BR-3 [SKBR3], a human breast adenocarcinoma cell line, showed that breast cancer cells showed that estrogen is the natural ligand of GPER; the cytomembranes of cells lacking ERα and ERβ expression but expressing GPER showed high-affinity, limited capacity, displaceable, and specific binding to estradiol-17β, and the binding was affected by an increase or decrease in GPER expression with progesterone stimulation and RNA interference, respectively (18). This evidence concerns the gene GPER1 and breast cancer.