This gene encodes a member of the steroid receptor superfamily. The encoded protein mediates the physiological effects of progesterone, which plays a central role in reproductive events associated with the establishment and maintenance of pregnancy. This gene uses two distinct promotors and translation start sites in the first exon to produce two isoforms, A and B. The two isoforms are identical except for the additional 165 amino acids found in the N-terminus of isoform B and mediate their own response genes and physiologic effects with little overlap. The location of transcription initiation for isoform A has not been clearly determined.