Prolactin (PRL), also known as luteotropic hormone or luteotropin, is a protein that in humans is best known for its role in enabling female mammals to produce milk, however, it is influential over a large number of functions with over 300 separate actions of PRL having been reported in various vertebrates. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation, and nursing. Prolactin is secreted in a pulsatile fashion in between these events. Prolactin also plays an essential role in metabolism, regulation of the immune system, and pancreatic development. Discovered in non-human animals around 1930 by Oscar Riddle at Cold Spring Harbor Laboratory on Long Island, New York, and confirmed in humans in 1970 by Henry Friesen prolactin is a peptide hormone, encoded by the PRL gene. Although often associated with human milk production, prolactin plays a wide range of other roles in both humans and other vertebrates. (For example, in fish—the oldest known vertebrates—an important function is probably related to control of water and salt balance.) Prolactin also acts in a cytokine-like manner and as an important regulator of the immune system. It has important cell cycle related functions as a growth-, differentiating- and anti-apoptotic factor. As a growth factor, binding to cytokine like receptors, it also has profound influence on hematopoiesis, angiogenesis and is involved in the regulation of blood clotting through several pathways. The hormone acts in endocrine, autocrine, and paracrine manner through the prolactin receptor and a large number of cytokine receptors. Pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus, which secrete dopamine (aka Prolactin Inhibitory Hormone) to act on the D2 receptors of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing factor (thyrotropin-releasing hormone) has a stimulatory effect on prolactin release. Several variants and forms are known per species. Many fish have variants prolactin A and prolactin B. Most vertebrates including humans also have the closely related somatolactin. In humans, three smaller (4, 16, and 22 kDa) and several larger (so called big and big-big) variants exist.