Progesterone receptor is unique in terms of the existence of two isoforms of A and B. The isoforms are expressed by two distinct promoters in the peripheral as well as the brain. The 5′-untranslated region and the coding region around ATG 1 of rat progesterone receptor form B as well as the ligand binding domain are cloned. The function of the forms A and B is still unclear. However, it has been reported that the form A is a hor-mone-dependent transdominant repressor of the B function under certain circumstance of the cell. This model is very interesting to elucidate the molecular mechanism of progesterone action.