Adjacent regions of mouse Chromosome 10 (MMU10) show conserved synteny with human chromosome 22 (HSA22) and the telomeric region of HSA21. Physical mapping on MMU10 using YAC fragmentation and PAC contig analyses demonstrates that Prmt2 has a position consistent with its human homolog, HRMT1L1, being telomeric to S100B on HSA21. This result establishes Prmt2 as the new proximal boundary of the region of conserved synteny between MMU10 and HSA21 and predicts that it is the most telomeric gene known on HSA21. Physical mapping refines the positions and order of HSA22 homologs Mmp11, Mif, and Ddt, demonstrates the orientation of S100b on the mouse chromosome, and localizes the junction of conserved synteny between HSA21 and HSA22 on MMU10. Comparative mapping in this region is important for defining gene structure and dosage imbalance in Down syndrome (DS), for developing animal models of DS, and for understanding processes of chromosome evolution.