HR+ breast cancer is highly heterogenous; it can be subdivided into the classifications of luminal A and luminal B. While luminal A breast cancer is estrogen and progesterone receptor positive with HER2 receptor negativity and low proliferation index as measured by Ki67, luminal B breast cancer often has lower hormone receptor sensitivity, higher proliferation index and can be either HER2 positive or negative. The gene discussed is ERBB2; the disease is breast carcinoma.