ESR1 and breast cancer: Based on gene expression profile, breast cancer can be classified into at least three subtypes: luminal tumors, which are positive for estrogen (ER+) and/or progesterone receptors (PR+); HER2-enriched, which overexpress the ERBB2 oncogene; and triple-negative tumors (TNBC), which lack of hormone receptors and HER2 amplification.