The human progesterone receptor (PR), is a ligand-activated transcription factor and is a member of the steroid receptor family. PR exists in humans as two isoforms; PR-A (94 kDa) which lacks the first 164 amino acids of PR-B and PR-B (114 kDa). While the two forms of PR have similar DNA- and ligand-binding affinities they have opposite transcriptional activities. PR-B functions as an activator of progesterone-responsive genes, while PR-A functions as a strong transdominant repressor of PR-B. This anti-PR recognize both PR-A and B. It labels epithelial cells of breast, ovary and endometrium.