OBJECTIVE Wolfram syndrome 1 (WS1) is an autosomal recessive disorder characterized by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness (DI DM OA D syndrome) associated with other variable clinical manifestations. The causative gene for WS1 (WFS1) encoding wolframin maps to chromosome 4p16.1. Wolframin has an important function in maintaining the homeostasis of the endoplasmic reticulum (ER) in pancreatic β cells. Recently, another causative gene, CISD2, has been identified in patients with a type of Wolfram syndrome (WS2) resulting in early optic atrophy, diabetes mellitus, deafness, decreased lifespan, but not diabetes insipidus. The CISD2-encoded protein ERIS (endoplasmic reticulum intermembrane small protein) also localizes to ER, but does not interact directly with wolframin. ERIS maps to chromosome 4q22. RESULTS Numerous studies have shown an interesting similarity between WFS1 and CISD2 genes. Experimental studies demonstrated that the Cisd2 knockout (Cisd2) mouse shows premature aging and typical symptoms of Wolfram syndrome. These researches provide interesting insight into the relation of neurodegenerative diseases, mitochondrial disorders, and autophagy and are useful for the pathophysiological understanding of both Wolfram syndrome and mitochondrial-mediated premature aging. CONCLUSIONS The knowledge of WS1 and WS2 pathogenesis, and of the interactions between WFS1 and CISD2 genes, is useful for accurate diagnostic classification and for diagnosis of presymptomatic individuals.