A new method for predicting susceptibility to insulin-dependent diabetes, or distinguishing between type-1 and type-2 diabetes, involves detecting the presence of antibodies in serum which bind to insulinoma-associated antigen-2-beta (IA-2-beta) and/or GAD65, by RIA, ELISA, depletion ELISA or immunoprecipitation. The method preferably uses recombinant IA-2-beta, produced in a baculo virus expression system. A new method for prevention or therapy of insulin-dependent diabetes involves immunization with an IA-2-beta recombinant vaccine. DNA encoding mammal IA-2-beta, or a hybridizing or homologous sequence, is also new. The DNA may be inserted in a vector with a promoter and terminator for expression in a host. The protein may be used in diagnostic, therapeutic and prophylactic compostitions for insulin-dependent diabetes.