ESR1 and breast cancer: In our ERα+ cell lines (MCF7, T47D) ERβ activation results in the increase expression of p21WAF/CIP1 and p27kip indicating that in ERα+ cell lines, ERβ provides and anti-proliferative role and agrees with overall BC RFS and other studies showing the anti-proliferative role of ERβ in ERα+ tumors [14–16, 20, 68, 69].