The prolactin receptor (PRLR) exists in all vertebrates in many isoforms, soluble or membrane-bound. It is expressed in a wide variety of tissues, and is responsible for the transmission of almost 300 distinct functions of its ligand, prolactin. This hormone activates the prolactin receptor by inducing its homodimerization, the first step required for triggering signaling cascades. No other membrane chain is required for signaling. Isoforms of prolactin receptor that vary in the length and sequence of the cytoplasmic tail have been identified; These variations account for the different signaling properties of the different isoforms. The JAK/STAT pathway (and mainly JAK2/STAT5) is the major signaling cascade. though many other proteins are also activated such as the MAP kinase pathway. Target genes are multiple, and confer to prolactin its numerous properties, such as lactogenic actions (milk protein gene induction) and mitogenic or antiapoptotic effects. Although no genetic abnormality has been linked to the PRLR gene, the involvement of this receptor in breast cancer has been proposed.