ESR1 and breast cancer: Based on the gene expression of the standard molecular markers, estrogen receptor α (ERα), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), breast cancer is categorized in four main subtypes: Luminal A (ERα and/or PR positive and HER2 negative); Luminal B (ERα and/or PR positive and HER2 positive or negative); HER2-positive (ERα/PR negative and HER2 positive); Triple-negative, TNBC (ERα/PR/HER2 negative), each characterized by different prognosis and response to drug treatment [3,4].