Retinol-binding protein 4 (RBP4) belongs to the apolipoprotein family and is a carrier specific for retinol (vitamin A alcohol) in the blood. It transports retinol from the liver to peripheral tissues. Recently, RBP4 has been recognized as an adipokine that contributes to insulin tolerance in AG4KO mice. It is secreted by adipocytes and signals other cells when blood glucose concentrations fall.Mutations in the RBP4 gene have been associated with autosomal dominant microphthalmia, anophthalmia, and ocular tissue mutilation.