Quantitative trait loci mapping was used to identify the chromosomal location of genes that contribute to increase the resistance to apoptosis induced in immature CD4+8+ thymocytes. An F2 intercross of the nonobese diabetic (NOD) mouse (displaying an apoptosis-resistance phenotype) and the C57BL/6 mouse (displaying a nonresistance phenotype) was phenotypically analyzed and genotyped for 32 murine microsatellite polymorphisms. Maximum likelihood methods identified a region on the distal part of chromosome 6 that is linked to dexamethazone-induced apoptosis (lod score = 3.46) and accounts for 14% of the phenotypic variation. This chromosomal region contains the diabetes susceptibility locus Idd6, suggesting that the apoptosis-resistance phenotype constitutes a pathogenesis factor in IDDM of NOD mice.