ESR1 and breast cancer: Data collected in ERα negative BC have also provided confounding results: some studies have shown a positive effect of ERβ1 presence, describing a reduced BC growth and invasiveness [23], and associating its high expression to good tamoxifen responsiveness [54] and to increased disease-free survival (DFS) or OS [98]; other studies indicate the negative effects of ERβ1 [30,99]; furthermore, there are studies claiming that ERβ1 presence has no prognostic significance (noteworthy, some of these results have been obtained with non-validated antibodies) [64,100,101].