Therefore, at least four breast cancer subgroups can be identified: Luminal A and Luminal B (which are positive for the hormone receptors), HER2 (which overexpresses the HER2 growth factor), and Basal-like (which is triple negative: estrogen receptor-negative (ER-), progesterone receptor-negative (PR-), and HER2-negative (HER2-). The gene discussed is ESR1; the disease is breast cancer.