Prolactin (PRL) is a secreted neuroendocrine hormone that acts primarily on the mammary gland to promote lactation. It is secreted by the anterior pituitary under tonic inhibition by hypothalamic dopamine. In humans Prolactin is also produced peripherally, while in rodents, the only known non-pituitary source is the placenta. Prolactin bioactivity is regulated by post-translational modifications including polymerization, phosphorylation, and proteolytic cleavage. Cleavage by matrix metalloproteinases or Cathepsin D can produce antiangiogenic fragments also called vasoinhibins.