With the exception of tumor grade, all other parameters determine the classification of tumors into molecular subtypes such as Luminal A (ER+, PR+, ki-67low, HER2−), Luminal B1 (ER+, PR− or ki-67high, HER2−), Luminal B2 (ER+, PR any, ki-67any, HER2+), HER2-enriched (ER−, PR−, ki-67any, HER2+) and Triple-Negative (ER−, PR−, ki-67any, HER2−), which are central to therapeutic decision-making and prognosis. The gene discussed is ESR1; the disease is neoplasm.