ESR1 and breast cancer: We compared three BC entities with different PD-L1 levels, namely (i) JIMT-1 cells (HER2-pos., trastuzumab resistant, PD-L1-high), (ii) MDA-MB-231 cells (TNBC, PD-L1 moderate), and (iii) MCF-7 cells (ER-pos./HER2-neg., PD-L1 very low).