The human Progesterone Receptor (PgR) is a member of the steroid family of nuclear receptors. PgR is found as a 120 kDa protein (Form B) or a 94 kDa protein (Form A) due to the use of alternative translation initiation sites. In its inactive state, PgR forms a multiprotein complex which includes heat shock proteins and immunophins. Upon binding of progesterone hormone to its receptor, there is a conformational change that allows dimerization and binding of the receptor to progesterone response elements (PRE) sequences, resulting in activated transcription.