Retinol-binding protein 4 (RBP4), as a sole retinol transporter in blood, is secreted from adipocytes and liver. The essence of RBP4 is a small molecule combination protein with hydrophobic properties, which is one of many transgenic proteins with ability to transport fat-soluble vitamins. RBP4 has been a drug target for ophthalmology research due to its role in vision. It may also be involved in metabolic diseases.

Adipocyte-specific deletion of Gluts led to notable elevation of RBP4 causing systemic insulin resistance, and that reduction of RBP4 improved insulin resistance. It identifies an important role of RBP4 in regulating insulin action and RBP4 is recorded as an adipocyte-derived hormone. Thus, measurement of serum or plasma RBP4 is a useful method for understanding metabolic disorders.