BrCa is a heterogeneous disease with multiple intrinsic tumor subtypes evidenced by the joint expression of molecular tumor markers such as estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor 2 (HER2, ERBB2) and a proliferation index (Ki67), based on their presence or absence; together with tumor size, tumor grade and nodal status [5]. The gene discussed is ESR1; the disease is invasive breast carcinoma.