GPER1 and breast cancer: Some studies 11 , 32 have reported pro-proliferative effects of G-1 and ICI in SKBr3 cells, showing that the absence of ERs, particularly ERα, and the presence of GPER can determine proliferative effects in estrogen-receptor-negative breast cancer cells, which explains why treatment with antiestrogens, such as tamoxifen and ICI, is not effective in this type of breast cancer.