The prolactin receptor (PRLR) is a member of the class I cytokine/lactogen receptor family which mediates the diverse cellular actions of prolactin in several tissues. PRLRs are expressed in normal and neoplastic human breast tissue, and in most breast cancer cells. PRLR contains an extracellular region that binds prolactin, a transmembrane region, and a cytoplasmatic region required for the activation of the Jak2–Stat5 signal transduction pathway by Prl which is essential for transcriptional activation of all known prolactin regulated genes. PRLRs have also been observed in ovarian follicular cells of mice, pigs, sheep, deer, and humans, as well as in luteal tissue in cow and horse ovaries. Furthermore, PRLR knockout mice exhibit failure of embryonic implantation, reduced number of mature oocytes, and low fertilization rates. Knockout females also display a reduced number of primary follicles.