The effects of progesterone are mediated by two functionally different isoforms of the progesterone receptor (PR), PR-A and PR-B, which are transcribed from distinct, estrogen inducible promoters within a single copy of the PR gene. Both PR-A and PR-B are ligand activated, but differ in their relative ability to activate target gene transcription. An inhibitory domain (ID), which maps to the amino terminus of the receptor, exists within both PR isoforms. Interestingly, the ID is functionally active only in PR-A and is necessary for steroid hormone transrepression by PR-A, suggesting that PR-A and PR-B may have different conformations in the cell.
