CCT4 and type 2 diabetes mellitus: Of interest, the major features determining islet progression from NGT to IGT and type 2 diabetes were ER stress genes, such as PDIA1, XBP1 and GRP78, while MAFA, NKX6.1 and CCT4 represented the main determinants of islet functional phenotype by directing the distribution of individual islets in the opposite direction (Fig. 4d).