We generated transchromosomal (Tc) mice containing a human chromosome 21 fragment (hCF21) using mouse embryonic stem (ES) cells with the transferred hCF21. Here we report breeding analyses that test the maintenance rate of the hCF21 in Tc mice of two different genetic backgrounds, MCH (ICR) and C57BL/6. Fluorescence in situ hybridization and polymerase chain reaction-based DNA analyses revealed that the structure of the hCF21 fragment including the CBR1, SIM2, HLCS, and D21S268 markers, was approximately 5 Mb in size, and was transmitted at least to the F3 generation. Though the retention rate of the hCF21 was variable among individual mice, for example, 21%-92% in brain and 10%-92% in tail fibroblasts, the C57BL/6 background yielded a higher retention rate than did the MCH (ICR). These results suggest that the hCF21 could be maintained stably in Tc mice, depending on the genetic background. The panel of Tc mice will be a useful model to investigate the function of genes on the hCF21 fragment in various tissues through germinal transmission.