As a transmembrane receptor, the prolactin receptor (PRL-R) interacts with prolactin. In addition, the PRL-R also binds and is activated by growth hormone (GH) and human placental lactogen (hPL). Preclinical investigations, epidemiological studies and analyses of tissue specimens from patients strongly support the contribution of prolactin receptor (PRLR) signaling to breast and prostate tumorigenesis and cancer progression. Moreover, The PRLR has been found to be essential for lobuloalveolar maturation of the mammary glands during pregnancy, as evidenced by the fact that PRLR knockout mice show severely impaired development of lobuloalveolar structures.