ESR1 and breast cancer: BC is a heterogeneous disease classified into clinically-relevant subtypes by immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH) detection of estrogen (ER) and progesterone (PR), configuring the hormone receptor (HR)-positive breast cancers, and human epidermal growth factor receptor 2 (HER2) receptors or ERBB2 gene amplification as HER2-positive breast cancers, or lastly the lack of these receptors’ expression, categorizing the triple-negative breast cancers (TNBC)2–4.