RBP4 is a specific carrier for retinol in the blood. It is a member of the lipocalin protein family. Under physiological conditions, RBP binds transthyretin (TTR) to prevent glomerular filtration of low molecular weight RBP in the kidneys. The RBP/TTR molar ration provides an indirect way to indicate marginal vitamin A deficiency. RBP4 may also play a role in insulin resistance in diabetes.