Susceptibility to murine and human insulin-dependent diabetes mellitus correlates strongly with major histocompatibility complex (MHC) class II I-A or HLA-DQ alleles that lack an aspartic acid at position β57. I-A^g7 lacks this aspartate and is the only class II allele expressed by the nonobese diabetic mouse. The crystal structure of I-A^g7 was determined at 2.6 angstrom resolution as a complex with a high-affinity peptide from the autoantigen glutamic acid decarboxylase (GAD) 65. I-A^g7 has a substantially wider peptide-binding groove around β57, which accounts for distinct peptide preferences compared with other MHC class II alleles. Loss of Asp^β57leads to an oxyanion hole in I-A^g7 that can be filled by peptide carboxyl residues or, perhaps, through interaction with the T cell receptor.