Wolfram syndrome is an autosomal recessive disorder characterized by juvenile diabetes mellitus, diabetes insipidus, optic atrophy and a number of neurological symptoms including deafness, ataxia and peripheral neuropathy. Mitochondrial DNA deletions have been described in a few patients and a locus has been mapped to 4p16 by linkage analysis. Susceptibility to psychiatric illness is reported to be high in affected individuals and increased in heterozygous carriers in Wolfram syndrome families. We screened four candidate genes in a refined critical linkage interval covered by an unfinished genomic sequence of 600 kb. One of these genes, subsequently named wolframin, codes for a predicted transmembrane protein which was expressed in various tissues, including brain and pancreas, and carried loss-of-function mutations in both alleles in Wolfram syndrome patients.