PGR and breast cancer: A study by Merki-Feld and colleagues reveals an increased cell proliferation rate in two estrogen and progesterone receptor-positive BC cell lines (ZR75-1 and HCC1500) following a long-term regimen with Estradiol and Ethynilestradiol, as well as a negative influence of estrogens on the expression of proliferative and apoptotic biomarkers such as elevated anti-apoptotic protein Bcl-2, and proliferating cell nuclear antigen (PCNA) [55].