The progesterone receptor (PR) is an intracellular steroid receptor that specially binds progesterone in humans. PR has been a member of the steroid receptor superfamily. It is encoded by a single PGR gene residing on chromosome 11q22. 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. It can be observed in human breast tissues. The proteins function as dimeric molecules in nuclei to regulate the transcription of target genes in a ligand-responsive manner.