Retinol binding protein (RBP) is a single-chain polypeptide glycoprotein, belongs to the alpha-globulin family of human plasma proteins and is the primary plasma transport protein for retinal. It binds retinal in a 1:1 stoichiometry, serving not only to solubilize retinal but also to protect it from oxidation. Once in circulation, the RBP-retinol complex binds to one molecule of transthyretin, which is a plasma thyroxine-binding protein. This complex then delivers retinal to specific receptors of the retina, skin, gonads, lungs, salivary glands, and other tissues.