BC subtypes are based on the expression of hormone receptors, i.e. estrogen receptor (ER) and/or progesterone receptor (PR) (∼75% of cases), and overexpression/amplification of the human epidermal growth factor receptor 2 (HER2) (∼20% of cases, half of which are also positive for hormone receptors). The gene discussed is PGR; the disease is breast cancer.