RBP4 and diabetes mellitus: Elevated circulating RBP4 levels were found to contribute to systemic insulin resistance and type 2 diabetes mellitus (T2DM) by inhibiting phosphatidylinositol 3 kinase (PI3K) activity in the skeletal muscles and promoting phosphoenolpyruvate carboxylase (PEPCK) expression in the liver of mouse models, which was first reported by Yang in 2005 [4].