IGFBP3 and type 2 diabetes mellitus: The proteins displayed by cluster 3 were significantly down-regulated from Ctrl to T2DM-nMCI and T2DM-MCI (Figure 1C), including learning- or memory-related proteins insulin-like growth factor-binding protein 3 (IGFBP3), beta-glucuronidase (GUSB), and corticotropin-releasing factor-binding protein (CRHBP), involving the secretion process (Figure 1D).