Prolactin is a hormone that is secreted by the pituitary gland. Besides the pituitary origin, prolactin is found in other tissues and cells such as adipocytes, immune cells, skin, prostate, brain, and the decidua. It is best known for its role in enabling female mammals to produce milk. However, prolactin also plays essential roles in the regulation of immune functions, pancreatic development, angiogenesis, reproduction, osmoregulation, and metabolism. This hormone is produced in increasing amounts during pregnancy and suckling, and it acts on the mammary gland by initiating and maintaining lactation in the postpartum period. Mouse prolactin has a 61% amino acid sequence homology with its human counterpart. In a knockout animal study, it was shown that female mice lacking the expression of prolactin are infertile. Nevertheless, they manifest spontaneous maternal behaviors. In addition, these animals show no sign of alterations in myelopoiesis and lymphopoiesis, suggesting that prolactin may not play an indispensable role in lymphocyte and myeloid development and myeloid differentiation. Epidemiological studies have demonstrated that circulating high levels of prolactin increase the risk of breast cancer. Prolactin may promote mammary tumor initiation via the JAK2/STAT5 signaling pathway.