Prolactin (PRL) is a single-chain polypeptide of 226 amino acids secreted by the anterior pituitary. It plays a role in multiple processes including cell growth, reproduction, and immune function. Anti-prolactin reacts with prolactin-producing cells and is a useful marker in classification of pituitary tumors and the study of pituitary diseases. It also plays a role in the development of mammary cancer, functioning dually as a mitogen and a differentiating agent.