Several important conclusions can be drawn from these experiments and are as follows. First, not only the lack of I-E but also the presence of unique I-A are involved in the development of autoimmune insulitis in a NOD mouse. Second, the mechanism of prevention in I-Ak transgenic mice might be different from that in I-E transgenic mice, although the negative selection of autoreactive T cells in thymus has been suggested in I-E transgenic mice. Third, the single amino acid substitution from Asp to Ser at position 57 of the Aβ chain is not sufficient for development of insulitis. Other amino acids including residue 56 of the Aβ chain as well as Aα chain are important in the development of insulitis and diabetes. However, the amino acid at position 57 may be involved in the conformational change of I-A molecule in such a way as to alter the interaction with the T cell receptor, leading to the positive or negative selection of autoreactive T cells. Fourth, the ectopic expression of class II molecules on β cells may not be the initial event for the generation of insulitis.