ESR1 and breast neoplasm: However, hormone-sensitive luminal breast tumors, which are estrogen receptor (ER) positive, progesterone receptor (PR) positive or negative, and human epidermal growth factor receptor 2 (HER2) negative, still represent a challenging subtype for oncologists, especially the more aggressive, highly proliferative so-called luminal B-like subtype, which is associated with a poorer prognosis than the more quiescent luminal A-like [1,2].