ESR1 and breast cancer: Breast cancers categorized according to molecular subtypes are defined as luminal A-like (ER positive and/or PR positive and HER2 negative), luminal B-like (ER positive and/or PR positive/PR negative and HER2 positive), HER2-enriched type (ER negative, PR negative, and HER2 positive), and basal-like (ER, PR, and HER2 negative).