ESR1 and breast cancer: There are four intrinsic subtypes of breast cancer: luminal A (estrogen receptor ER+, progesterone receptor PR+, and human epidermal growth factor receptor 2 HER2), with a KI67 index of less than 14%; luminal B has a KI67% of more than 14% (ER+, PR+, HER2- or HER2+); HER2-enriched (ER-, PR-, and HER2 overexpression); and basal-like or triple-negative breast cancer (TNBC), which lacks the three hormonal receptors (ER-, PR-, and HER2-) [2–4].