The progesterone receptor (PR, also known as NR3C3), is a member of the steroid receptor superfamily, mediates the physiologic effects of progesterone. It is activated by the steroid hormone progesterone. In humans, PR is encoded by a single PGR gene residing on chromosome 11q22, it has two main forms, A and B, that differ in their molecular weight. The human progesterone receptor exists as 2 functionally distinct isoforms, PRA and PRB. PRB functions as a transcriptional activator in most cell and promoter contexts, while PRA is transcriptionally inactive and functions as a strong ligand-dependent transdominant repressor of steroid hormone receptor transcriptional activity.