The prolactin receptor (PRLR) is a type I cytokine receptor encoded in humans by the PRLR gene on chromosome 5p13-14. It is the receptor for prolactin (PRL). The PRLR can also bind to and be activated by growth hormone (GH) and human placental lactogen (hPL). The PRLR is expressed in the mammary glands, pituitary gland, and other tissues. It plays an important role in lobuloalveolar development of the mammary glands during pregnancy and in lactation. The PRLR is a class 1 cytokine receptor that uses messenger pathways to control cell proliferation, migration, intracellular ion concentration and inhibit programmed cell death (apoptosis). Expression of the PRLR protein is found within cells of the mammary glands[10] in accordance with its role in lactation, but also is the subject of attention for its diverse and emerging roles by its expression in adipose tissue, pancreatic islet cell proliferation, and immune responses. 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. Disruption of PRLR signaling pathways have been linked to tumorigenesis and breast cancer development.