The strong correlations of RBP4 levels with the levels of fasting glucose and HbA1c and HOMA-β index as well as the correlation of relative ratios based on RBP4 levels with QUICKI and HOMA-β indexes suggest that RBP4 plays an important role in β-cell dysfunction and has a potential diagnostic value for T2DM. This evidence concerns the gene RBP4 and type 2 diabetes mellitus.