Invasive breast cancer that expresses the two hormone receptors (HRs), estrogen receptor (ER), and progesterone receptor (PR) and presents a negative HER2 status and low expression level of the proliferation marker Ki-67 (+/− low or intermediate tumor grade) is classified as luminal A-like tumors; HR-positive, HER2-negative breast cancer with a high Ki-67 level +/− high tumor grade (G3) is classified as the luminal B-like subtype. The gene discussed is ESR1; the disease is breast cancer.