ESR1 and breast cancer: The classifications of breast cancer are commonly used as Receptor Status-Based by Immunohistochemistry (IHC), that is Hormone receptor-positive (ER+ and/or PR+), HER2-positive and Triple-negative (ER−, PR−, HER2−), as well as Molecular Subtype Classification (Luminal A/B, HER2-enriched and Basal-like) [67].