ESR1 and breast cancer: The current immunohistochemistry-based molecular classification divides breast cancers into four subtypes—Luminal A-like (hormone receptor-positive/low proliferation), Luminal B-like (hormone receptor-positive/high proliferation), HER2-positive (estrogen receptor negative, ERBB2-amplified), and triple-negative (TNBC, based on the absence of estrogen receptor, progesterone receptor and HER2 expression) [3].