In order to decide on treatment regimens, breast cancer patients are clinically classified according to the expression levels of the following molecular markers: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2 also known as ERBB2) as well as the proliferation marker Ki-67 [29,30,31,32]. The gene discussed is PGR; the disease is breast cancer.