The prolactin receptor (PRLR), a cytokine receptor — encoded by a gene on chromosome 5p13-14 — interacts with prolactin as a transmembrane receptor. Thus it contains an extracellular region that binds prolactin, a transmembrane region, and a cytoplasmatic region. PRLR is widely expressed in breast, placenta, kidney, liver and pancreas and also activated by growth hormone (GH) and human placental lactogen (hPL) in addition to prolactin. It has been well documented that abnormity of PRLR is closely related to the pathogenesis, progression and prognosis of cancers including breast cancer. Specific binding to PRLR can stimulate the breast cancer cells to grow and certain antagonists which bind but do not dimerize PRLR can inhibit the growth of breast cancer cells. Several isoforms have been described, and the soluble form are unable to transduce prolactin signaling.