ESR1 and breast cancer: Based on the expression status of the hormone receptors estrogen receptor (ER), progesterone receptor (PR), and the human epidermal growth factor receptor (HER2), breast cancer can be categorized into Luminal A, Luminal B, HER2-overexpressing/enriched, triple-negative/basal-like breast cancer (TNBC), and normal-like breast cancer (NLBC) [36,37,38,39,40,41,42,43] (Figure 2, Table 1).