The invention relates to a mutation in the Dock8 encoding gene that is causative for type 1 diabetes mellitus at least in the rat (rattus norvegicus) or mouse, the rat or mouse preferably having an MHC predisposing towards type 1 diabetes mellitus and/or a mutation in the von Willebrand factor Vwa2. Embodiments of the invention comprise a process for generating a non-human mammal by genetic manipulation to contain at least heterozygously, preferably homozygously, the mutation causative for type 1 diabetes mellitus, preferably in combination with an MHC predisposing for type 1 diabetes mellitus and/or with a mutation in the von Willebrand factor Vwa2.
