Type 1 diabetes is a genetically determined disorder, with an increased incidence in monozygotic twins and first-degree relatives or people with type 1 diabetes. Approximately 70% of monozygotic twins develop type 1 diabetes (with prolonged follow-up), and a first degrees relative of a person with type 1 diabetes has approximately one chance in twenty (5% risk) of developing the disease (vs. 1:300 in the general population). The responsible genes are within the major histocompatability complex (MHC) located on chromosome 6 (also called the HB locus). About 40% of the familial aggregation of autoimmune type 1 diabetes is explained by MHC genes, especially HLA class II molecules DQ and DR. Ninety-five percent of type 1 diabetics carry HLA D3, Dl or both compared with 45% of the general population. The presence! an aspartic acid residue at position 57 of the DQ 3 chain is protective for the development of type 1 diabetes. Clustering of long-term complications in families studied in the DCCT suggests that a genetic component contributes to vascular complications.
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