ESR1 and breast cancer: The main molecular subtypes of breast cancer are luminal A (ER positive, PR positive and HER2 negative), luminal B (ER positive, PR negative and HER2 negative), HER2-enriched (ER negative, PR negative and HER2 positive), triple-negative breast cancer (TNBC), basal-like (ER negative, PR negative and HER2 negative), claudin low (ER negative, claudin negative, vimentin positive, E-cadherin low) and normal breast-like (adipose tissue gene signature) [7].