Progesterone receptor (PR) is a protein found inside cells, which is activated by the steroid hormone progesterone. In humans, PR is encoded by a single PGR gene residing on chromosome 11q22, it has two main forms, A and B, that differ in their molecular weight. Progesterone is necessary to induce the progesterone receptors. When no binding hormone is present the carboxyl terminal inhibits transcription. Binding to a hormone induces a structural change that removes the inhibitory action. Progesterone antagonists prevent the structural reconfiguration. After progesterone binds to the receptor, restructuring with dimerization follows and the complex enters the nucleus and binds to DNA. There transcription takes place, resulting in formation of messenger RNA that is translated by ribosomes to produce specific proteins. Progesterone receptor plays a central role in diverse reproductive events associated with establishment and maintenance of pregnancy, alveolar development in the breast and sexual behavior.