Recently, the rat Ian4 (rIan4) was cloned and it appears to be identical to the gene Iddm1/lyp responsible for severe lymphopenia and the development of insulin-dependent diabetes in the BB-DP rat.
Functional and genetic data support the hypothesis that the gene responsible for the lymphopenia, Lyp, is also a diabetes susceptibility gene, named Iddm1.
Analysis of cross hybrids of diabetic BB/OK rats and rats of different diabetes-resistant strains has demonstrated that beside the MHC genes, Iddm1 and the lymphopenia, Iddm2, additional non-MHC genes are involved in diabetes development.
Several crossing studies using diabetic BB/OK and diabetes-resistant rat strains have clearly shown that the MHC class-II-genes of the RT1u haplotype (Iddm1) and the lymphopenia (Iddm2) are essential but not sufficient for type 1 diabetes development.