Human serum retinol binding protein 4 -sRBP4) binds to one equivalent of vitamin A and is one of the major retinol carriers found in the blood of mammals. Humans RBP4 is a monomeric 21 kDa .beta.-sheet-rich protein that contains three disulfide bonds and belongs to the lipocalin protein family. sRBP4 is synthesized and sequestered in hepatocytes until retinol binding triggers its secretion . In plasma, sRBP4 typically forms a 1:1 complex with the 55 kDa tetrameric protein transthyretin -TTR) which prevents RBP from being removed from the plasma by glomerular filtration. Studies have shown that sRBP4 is an adipocyte-derived signal that may contribute to the pathogenesis of type 2 diabetes. Elevation of sRBP4 causes systemic insulin resistance whereas reduction of serum concentrations improves insulin action.