Retinol-binding protein 4 (RBP4) is a Lipocalin superfamily molecule that transports vitamin A (retinol) and retinaldehyde in the serum. Retinaldehyde is the critical chromophore in the rhodopsin photoreceptor, while both it and retinoic acid regulate a multitude of cellular differentiation and proliferation effects through the intracellular receptors RAR and RXR. RBP4 is secreted primarily by hepatocytes and adipocytes into the blood where the RBP4-retinol complex interacts with transthyretin (TTR). Formation of the complex with TTR increases the serum half-life of RBP4 by preventing RBP4 filtration through the kidney. The C-terminally processed forms of RBP4, which do not bind TTR, are normally excreted into the urine but accumulate in the serum during renal failure. RBP4 promotes hyperglycemia, and its upregulation in visceral and liver adipocytes leads to elevated serum levels.