ESR1 and breast cancer: Here we analyzed the constitutive PD-L1 expression and the cellular localization in subtype-specific BC cell lines as well as the cellular distribution of ER-positive/Her2-negative, ER/Her2 double-positive, and triple-negative BC cell lines under standardized conditions in vitro and as a function of cytotoxic treatments (i.e., Epirubicin [Epi], Paclitaxel [Ptx]).