Retinol binding protein 4, plasma, also known as RBP4, belongs to the lipocalin family and is the specific carrier for retinol(vitamin A alcohol) in the blood. It is protein that in humans is encoded by the RBP4 gene. RBP4 gene resides just centromeric of the cluster of CYP2C genes on 10q24. The mouse Rbp4 locus is closely linked and just proximal to the locus for phenobarbital-inducible cytochrome P450-2c(Cyp-2c) at the distal end of chromosome 19. It delivers retinol from the liver stores to the peripheral tissues. In plasma, the RBP-retinol complex interacts with transthyretin, which prevents its loss by filtration through the kidney glomeruli. A deficiency of vitamin A blocks secretion of the binding protein posttranslationally and results in defective delivery and supply to the epidermal cells. The standard used in this kit is recombinant protein, with E19-L201 aa sequence, the molecular weight is 22kda.