Retinol binding protein 4(RBP4), originally known as a specific transport of retinol in blood, is also a novel inflammatory and insulin resistance marker. Serum levels of RBP4 increased in insulin resistant and diabetes. Studies both in human and animal suggested that serum levels of RBP4 plays a key role in the link between obesity, insulin resistant and diabetes. Animal experiments found that increased secretion of RBP4 might reduces insulin-dependent glucose uptake by muscle tissue by reducing the activity of PI(3)K (phosphoinositide 3-kinase), and increased hepatic glucose output by increasing the expression of the enzyme PEPCK. However, whether serum RBP4 could be a biomarker of type 2 diabetes risk still remain unclear. SDS-PAGE purified.