Technical Abstract: Common enterovirus infections appear to initiate or facilitate the pathogenetic processes leading to type 1 diabetes (T1D), and also sometimes precipitate the clinical disease. In experimental infection of mice coxsackieviruses have shown to have propensity particularly to exocrine tissue while even in lethal cases the islets remain unaffected. The virus strain most intensively studied in this respect is the diabetogenic variant E2 of coxsackievirus B4. In addition, it is known that all six serotypes of coxsackie B viruses could be made diabetogenic by repeated passages in either mouse pancreas in vivo or in cultured mouse beta-cells in vitro. However, the genetic determinants of the phenomenon have not been determined. In the present study a laboratory strain of coxsackievirus B5 was passaged repeatedly in mouse pancreas in vivo. After 15 passages the virus phenotype was clearly changed and infection of the variant resulted in a diabetes like syndrome in mice characterized by chronic pancreatic inflammation together with transient increases in blood glucose if measured after glucose tolerance, loss of pancreatic acinar tissue, and mild insulitis. In order to found out genetic determinants involved in mouse pancreas adaptation the passaged virus variant together with the parental virus strain was cloned for molecular characterization. The whole genome sequencing of both virus strains revealed only limited differences. Altogether, eight nucleotides were changed resulting in five amino acid substitutions out of which three were located in capsid proteins.