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Three groups of patients with insulin-dependent diabetes mellitus, ascertained by different procedures, were investigated for HLA-A, B, C and D antigens (n = 164), and a subset (n = 93) for HLA-DR. Both HLA-D/DR3 and D/DR4 were strongly positively associated and D/DR2 was negatively associated with insulin-dependent diabetes. HLA-DR+ was found to be a better marker for insulin-dependent diabetes than Dw4. The HLA-B associations (B8, B15 and B18) were clearly secondary to the increases of HLA-D/DR3 and D/DR 4. The HLA associations did not differ between familial and isolated cases indicating that these two groups may well have a common genetic background. Based on analysis of HLA-haplotype sharing in affected sibling pairs, a simple dominant model of inheritance could be ruled out, and a simple recessive model was found unlikely. The relative risks for the HLA-Dw3,4 and HLA-DR3,4 phenotype were 21.2 and 44.4 respectively and exceeded those of both the HLA-Dw3 and HLA-DR3 (5.6 and 4.3) as well as the HLA-Dw4 and DR4 (10.1 and 10.5) phenotypes. This argues against an intermediate genetic model but further studies are needed to clarify whether there is more than one susceptibility gene for insulin-dependent diabetes mellitus within the HLA-system.

There are marked geographic differences in the incidence of insulin-dependent diabetes mellitus (IDDM); for example, children in countries such as Finland are over 35 times more likely to develop IDDM than children in Japan. An understanding of the reasons for the geographic differences is likely to be important for understanding and, hopefully, preventing IDDM. There are problems, however, because of the lack of registries with adequate standardization. The major needs for the future studies include (1) to clarify the definition of IDDM for epidemiologic study, (2) to establish a standardized approach for IDDM registries, (3) to use registries to evaluate viral, immunologic, and genetic differences in order to explain differential risks across populations, and (4) to encourage the development of new population-based registries worldwide.