To better investigate this heterogeneity and to study, at the islet level, the molecular mechanisms accompanying beta cell dysfunction during progression to type 2 diabetes, we studied the expression of 14 selected phenotypic and ER stress-related genes (see the ESM Methods section) together with individual in situ immunofluorescence insulin and proinsulin data (a total of 17 markers) of individual microdissected pancreatic islets obtained from NGT, IGT and type 2 diabetic individuals (ESM Fig. 8). Here, INS is linked to type 2 diabetes mellitus.