Iddm 1 in the BioBreeding (BB) rat designates the genomic region on rat Chromosome (Chr) 4 that harbors the gene causing peripheral T cell lymphopenia (Lyp) and diabetes (Jacob et al. 1992). We here report the generation of a novel congenic Lyp/Lyp Dr rat line, in which iddm 1 has been introgressed onto the diabetes-resistant BB/Dr background. This was accomplished by marker-assisted selection (MAS) (Lande and Thompson 1990) in nine cycles of cross-intercross breeding of
diabetes-prone Dp with Dr BB rats. As a result, we have generated a congenic line in which diabetes and lymphopenia are controlled solely by iddm 1.