The effects of progesterone are mediated by two functionally different isoforms of the progesterone receptor, PR-A and PR-B, which are transcribed from distinct, estrogen inducible promoters within a single copy of the PR gene. The PR-A and PR-B proteins are 90 kDa and 118 kDa respectively; the first 164 amino acids of PR-B are absent in PR-A. Progesterone bound PR-A and PR-B have different transcription activation properties. Specifically, PR-B functions as a transcriptional activator in most cell and promoter contexts, while PR-A is transcriptionally inactive and functions as a strong ligand dependent transdominant repressor of steroid hormone receptor transcriptiona activity. An inhibitory domain (ID), which maps to the amino terminus of the receptor, exists within both PR isoforms.