ESR1 and breast cancer: Genetic profiling has revealed several subclasses of breast cancer based on hormone receptor (ER and PR) and HER2 (ERBB2) status: luminal ER-positive and PR-positive, which is further subdivided into luminal A and B; HER2-positive; and triple-negative breast cancer (TNBC; ER-ve PR-ve HER2-ve) [7].