Prolactin is a hormone that is produced and secreted by the pituitary gland. Prolactin acts in an endocrine, paracrine, and autocrine manner. The prolactin receptor (PRLR) is expressed on many cell types, including cells of the reproductive organs, central nervous system, and breast cancer. Prolactin signal transduction occurs via JAK kinase signaling pathways. The primary function of prolactin is to regulate lactation, but prolactin also plays functional roles in the immune system and during cell growth, apoptosis, and differentiation.