studied the mechanism of the exaggerated susceptibility to IDDM in DR3/DR4 heterozygotes, and concluded that its basis is the formation of hybrid molecules of the closely linked DQ-alpha (HLA-DQA1; 146880) and -beta (HLA-DQB1; 604305) chains. The DR-alpha molecules are not polymorphic, and mixed DR alpha-beta dimers would not result in novel HLA molecules. On the other hand, both the alpha and beta chains of DQ are polymorphic, and a DQ alpha-beta dimer composed of transcomplementing chains would be unique to a heterozygous individual and not expressed in either parent. In the mouse, such transcomplementation has been demonstrated structurally, and epitopes newly formed in the resulting hybrid molecules allow for an altered functional immune response different from that of either parent.