At the clinical level, breast cancer is classified into five main subtypes [luminal A, luminal B (HER2+), luminal B (HER2−), HER2, and triple negative] based on immunohistochemical assays for estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-678. The gene discussed is PGR; the disease is breast cancer.