ESR1 and breast cancer: The current breast cancer treatment options are determined by the combined status of three key receptors; estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), resulting in four molecular subtypes (Luminal A, Luminal B, HER2-enriched, and triple negative) which differ in their gene expression patterns, clinical features, treatment response, and prognosis (Schnitt, 2010).