Prolactin (PRL), also known as luteotropic hormone or luteotropin, is a protein that in humans is best known for its role in enabling mammals, usually females, to produce milk. It is influential over 300 separate processes in various vertebrates. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. Prolactin is secreted in pulses in between these events. Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development. In humans, prolactin is produced at least in the anterior pituitary, decidua, myometrium, breast, lymphocytes, leukocytes and prostate. Prolactin follows diurnal and ovulatory cycles. Prolactin levels peak during REM sleep and in the early morning. Many mammals experience a seasonal cycle. The structure of prolactin is similar to that of growth hormone and placental lactogen.