Progesterone plays a central role in the reproductive events associated with the establishment and maintenance of pregnancy. Progesterone receptor, a member of the steroid receptor superfamily, mediates the physiologic effects of progesterone. The PGR gene uses separate promoters and translational start sites to produce 2 isoforms, PRA and PRB, which are identical except for an additional 165 amino acids present only in the N terminus of PRB. Although PRA and PRB share several structural domains, they are distinct transcription factors that mediate their own response genes and physiologic effects with little overlap. It is composed of three domains: a modulating N terminal domain, a DNA binding domain and a C terminal steroid binding domain.