ESR1 and breast cancer: Depending on the molecular data and measuring techniques used, the subtyping of BC may differ slightly, but it is usually classified into several major subtypes based primarily on the status of 3 major cell-surface receptors: luminal A [estrogen receptor (ER) and/or progestogen receptor (PR) positive, HER2 negative]; luminal B (ER+ and/or PR+, HER2+); HER2-amplified (ER-, PR-, HER2+), and triple-negative BC (Basal-like or TNBC, ER-, PR-, HER2-)[12].