Progesterone receptor (PR) gene and main isoforms. (a) Exon organisation map of the PR gene, showing PR-A and PR-B promoters (indicated by arrows). (b) PR-B and PR-A protein structures. The A/B region is encoded by exon 1 and part of exon 2, and contains the PR-B specific transactivation domain AF-3, AF-1, common to PR-B and PR-A, and the PR-A specific inhibitory domain (ID). The C region forms the DNA binding domain (DBD); each of exons 2 and 3 encodes one zinc finger. The D region is encoded by exon 4 and part of exon 3, and forms the hinge region responsible for the nuclear location signal (NLS). The E region contains AF-2, common to PR-B and PR-A, and the hormone (ligand) binding domain (LBD), encoded by exons 4 to 8. (c) PR-C, an amino-terminally deleted PR protein predicted to result from alternative translation initiation at a methionine at position 595. PR-C lacks a complete DBD and the first two transactivation domains (AF-3 and AF-1).