Type 1 diabetes and celiac disease, both of which are associated with HLA class II genes, cosegregate in populations, suggesting a common genetic origin. In this article, the authors tested whether any non-HLA loci are shared. They report susceptibility alleles shared by both diseases, indicating that common biologic mechanisms underlie these immune-mediated disorders. Type 1 diabetes and celiac disease cosegregate in populations, suggesting a common genetic origin. These authors report susceptibility alleles shared by both diseases, indicating that common biologic mechanisms underlie these immune-mediated disorders. Type 1 diabetes is caused by autoimmune destruction of the insulin-producing beta cells in the pancreatic islets. The disease affects approximately 0.4% of persons of European origin and is strongly clustered in families. The major susceptibility genes — the HLA class II loci, HLA-DQB1 and HLA-DRB1 on chromosome 6p21 — act in combination with many other non-HLA loci across the genome, 1 , 2 with unknown environmental factors playing a major role. 3 – 6 Celiac disease, which results from an immune, inflammatory reaction in the small intestine to proteins in ingested barley, wheat, and rye gluten, occurs in approximately 0.1% of persons of . . .