GPER1 and breast cancer: GPR30 has been reported to be expressed in ER− breast cancer cells and suggested to be an excellent new therapeutic target for the treatment of ER− breast cancer.17 Confusedly, the only two published papers reported contradictory results: Girgert et al.18 stated that activation of GPR30 promotes growth of ER− breast cancer cells, while Weissenborn et al.19 revealed that GPR30 functions as a tumor suppressor of ER− breast cancer cells.