Retinol binding protein 4, also known as RBP4, is a transporter protein[5] for retinol (vitamin A alcohol). RBP4 has a molecular weight of approximately 21 kDa and is encoded by the RBP4 gene in humans. It is mainly, though not exclusively, synthesized in the liver and circulates in the bloodstream as a hepatokine bound to retinol in a complex with transthyretin. RBP4 has been a drug target for ophthalmology research due to its role in vision. RBP4 may also be involved in metabolic diseases as suggested by recent studies.