Breast cancer has a distinct tumor heterogeneity, with multiple subtypes and differences in incidence, treatment options and prognosis for each subtype such as distinct molecular subtypes based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor type 2 (HER2) with the use of immunohistochemistry: luminal A, luminal B, HER2 and triple-negative breast cancer (TNBC) [3, 4]. The gene discussed is ESR1; the disease is breast cancer.