BCa is a heterogeneous cancer that has four major molecular subtypes: Luminal A (estrogen receptor (ER)high, progesterone receptor (PR)high/low, human epidermal growth factor receptor 2 (HER2)low); Luminal B (ERhigh, PRhigh/low, HER2high/lowki-67high); HER2 enriched (ERlow, PRlow, Her2high); Triple-negative BCa (ERlow, PRlow and HER2low) [6]. The gene discussed is ESR1; the disease is cancer.