The disease is classified into distinct subtypes based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2): Luminal A, Luminal B, HER2, and triple-negative breast cancer (TNBC) (Goldhirsch et al., 2013). Here, ESR1 is linked to triple-negative breast carcinoma.