Prolactin (PRL) is a neuroendocrine pituitary hormone encoded by the PRL gene that is synthesized by the pituitary gland, placenta, brain, uterus, dermal fibroblasts, meconium, B cells, T cells, natural killer cells, and breast cancer cells. Prolactin is expressed at low levels in early human gestation, is replaced by placental prolactin, and increases in late gestation. Prolactin plays an important role in metabolism, immune system regulation, and pancreatic development. Prolactin also acts in a cytokine-like manner and is an important regulator of the immune system. Prolactin has important cell cycle-related functions as a growth, differentiation and anti-apoptotic factor. As a growth factor that binds to cytokine-like receptors, it affects hematopoiesis, angiogenesis, and the regulation of blood coagulation through a variety of pathways.