LEP and type 2 diabetes mellitus: In addition to leptin and adiponectin, which are well-established biomarkers for metabolic dysfunction [2, 4], retinol-binding protein 4 (RBP4), a transport protein for retinols, has been shown to play a role in establishing the link between obesity and IR, and thereby contributes to the pathogenesis of MS and type 2 diabetes (T2D) [5–9].