While it has been reported that the progestogens P4 (76, 77), MPA (76–79), NET (77–79) and DRSP (76) decrease E2-induced proliferation of the ER/PR positive HCC1500, T47D or MCF-7 breast cancer cell lines, others have shown that P4 (63, 80), MPA and NET (80) increase E2-induced cell growth of BT474 or T47D xenograft tumors in mice, and that P4 increases E2-induced anchorage-independent growth of MCF-7 cells (62). This evidence concerns the gene ESR1 and breast cancer.