Employing three distinct breast cancer cell lines—MCF-7 (ESR+; GPER+), MDA-MB-231 (ESR−; GPER−), and SkBr3 (ESR−; GPER+)—this study subjected them to treatment with two tamoxifen derivatives: 4-hydroxytamoxifen (4-HT) and endoxifen (Endox). This evidence concerns the gene GPER1 and breast carcinoma.