ESR1 and triple-negative breast carcinoma: The inverse correlation between ERα and PD-L1 was confirmed not only in the MMTV-PyMT transgenic mouse model [81] but also in human BC specimens, where the ratio of PD-L1 positive patients was much lower in ER+/HER2− BC (19.4%) than in triple-negative breast cancer (TNBC, 58.6%) [82, 83].