ESR1 and breast cancer: Previous studies have shown that PD-L1protein or mRNA expression levels in breast cancer tumor samples were associated with large tumor size, high tumor grade, more positive lymph-node involvement, ER-negative status, PR-negative status, ERBB2-positive status, and high proliferation, as well as unfavorable molecular subtypes such as HER2-enriched breast cancer or TNBC [12, 14, 15, 17, 24].