Prolactin, also named PRL and lactotrope is a peptide hormone, encoded by the PRL gene. The major form of PRL is a 23 kDa monomeric protein, splice variants of 14, 16, and 22 kDa have been identified. PRL has also been found to be glycosylated, phosphorylated, dimerized, and polymerized. Glycosylation, phosphorylation, dimerization, or polymerization of PRL result in lower activity. Rat prolactin shares 77 % ~ 90 % a.a. sequence identity with human and mouse prolactin. There are mainly three different forms of prolactin in regard to size: little prolactin, big prolactin and big big prolactin. Little prolactin is the predominant form. Although often associated with human milk production, prolactin plays a wide range of other roles in both humans and other verterbrates.