Progesterone is one of the central regulators of female reproduction. In breast development, progesterone is involved in the formation of lobular-alveolar structures and affects differentiation in the breast by modulation of milk protein synthesis. The cellular effects of progesterone are mediated through progesterone receptors (PR). PR, a protein with 946 amino acids, is a ligand-activated transcription factor member of the steroid receptor superfamily of nuclear receptors. PR is predominantly expressed in female sex steroid-responsive tissues such as the mammary gland, uterus, and ovary but is also found in other tissues such as endocrine cells of the Langerhans’ islets. The estrogen receptor (ER) and PR status have been used for over 20 years to predict breast carcinoma responsiveness to endocrine therapy and as a prognostic indicator for early recurrence.