PGR and breast cancer: Breast cancer has three major clinical subtypes that matter to clinicians, patients and families alike: 70% of breast cancers are hormone (estrogen and progesterone) receptor positive, 15–20% are human epidermal growth factor (ERBB2) positive (formerly HER2), and 15% are “triple negative” or do not possess receptors for homones or ERBB2 (5).