Gallen International Expert Consensus includestreatment algorithms based on the classification of breast cancer subtypes byimmunohistochemistry findings for estrogen receptor (ER), progesterone receptor (PR), HER2, andKi67 expression.14,15 Although breast cancer is a heterogeneous assembly of diseases, it can beclinically divided by hormone receptor, HER2, and Ki67 expression to guide therapeuticinterventions. The gene discussed is ESR1; the disease is breast carcinoma.