Using a cut-off of ≥5%, PD-L1 expression was noted in 32.8% (329/1009) of BC (Figure 1A,B) and found to be associated with adverse clinico-pathological parameters such as younger age (p = 0.0432), higher grade (p = 0.0025), ER-negative (p < 0.0001), PR-negative (p = 0.0001), and triple-negative (p = 0.0062) breast cancers, as well as a high proliferative index (Ki-67) (p < 0.0001). This evidence concerns the gene ESR1 and breast cancer.