Bole-Feysot, C., Goffin, V., Edery, M., Binart, N., & Kelly, P. A. (1998). Prolactin (PRL) and its receptor: Actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice. Endocrine Reviews, 19(3), 225-268.
Prolactin and GH (growth hormone), along with placental lactogens (PLs), form a family of hormones that probably result from the duplication of an ancestral gene. It was early in the 20th century that changes in the histology of the anterior pituitary gland of pregnant women were first noted. French researchers were the first to identify a pituitary factor capable of inducing milk secretion in rabbits (1). American scientists made similar observations, and in addition to naming the new pituitary factor prolactin, showed that prolactin was able to stimulate the growth of the pigeon crop sac (2). Prolactin has now been shown to exist in all vertebrates thus far examined.