Progesterone receptor A

The progesterone receptor A is one of three known isoforms of the progesterone receptor, the main biological target of the endogenous progestogen sex hormone progesterone. The other isoforms of the PR include the PR-B and PR-C.
PR-A is 164 residues shorter than PR-B in humans and anywhere from 128-165 residues shorter in other organisms. Each isoform binds its natural ligand, progesterone, but also demonstrates the ability to bind a number of other agonists including norethindrone, a synthetic progestin.

Expression and overexpression

PR-A and PR-B are generally expressed in equal ratios, but PR-A is expressed in larger amounts in uterine stromal cells normally. A spike in PR-A expression in the myometrium has been observed to initiate parturition in placental mammals.
PR-A is the isoform most commonly observed to be overexpressed in human breast cancer. Currently PR is estimated by immunohistochemistry and earlier was quantified by standardized radio-ligand binding assays developed by New England Nuclear and Wittliff. Patients with PR-A rich carcinomas, as opposed to patients with PR-B rich carcinomas, have faster recurrence rates.