INS and type 2 diabetes mellitus: Insulin sensitivity and glucose tolerance. Elevated RBP4 in the circulation of type 2 diabetic patients was reported many years ago (Basualdo et al., 1997; Abahusain et al., 1999) but it took until 2005, when a causal link between circulating RBP4 and insulin resistance was presented: Yang et al. showed, besides increased RBP4 in blood of multiple insulin resistant mouse models, that RBP4-deficient mice were less prone to develop insulin resistance (Yang et al., 2005).