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 beta 57. 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 (CAD) 65. I-A(g7) has a substantially wider peptide-binding groove around beta 57, which accounts for distinct peptide preferences compared with other MHC class II alleles. Loss of Asp(beta 57) Leads 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.