(PRL), also known as lactotrope, is a protein that in
humans is probably 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.
Discovered in non-human animals around 1930
by Oscar Riddle, 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.