Retinol binding protein 4 (RBP4) is a newly discovered adipose factor that is expressed and secreted in adipose tissue. RBP4 is not a newly discovered protein, but belongs to the retinol binding protein (RBP) family. Retinol binding proteins are a specific transporter protein in the body that transports vitamin A and its various active metabolites from the liver to target tissues, assisting in the physiological function of vitamin A. Based on sequence homology, retinol binding proteins can be divided into five categories: serum retinol binding protein (RBP), intracellular retinol binding protein (CRBP), intracellular retinol binding protein (CRABP), intracellular retinol binding protein (CRALBP), and photoreceptor retinol binding protein (IRBP). The latter four are all intracellular binding proteins, while RBP4 is a serum retinol binding protein, which is the main extracellular transport protein responsible for binding and transporting retinol in the blood.