A mammalian islet cell antigen polypeptide involved in the development of insulin-dependent diabetes mellitus (IDDM) is disclosed. This islet cell antigen polypeptide, 1851, was found to contain regions of homology to the protein tyrosine phosphatase family. Methods for diagnosis and treatment, including use in immunoprecipitation assays and the induction of immune tolerance using the recombinant mammalian polypeptides and antibodies specific to mammalian islet cell antigen 1851 polypeptides are presented.