Comparative maps define the HSA 21:22 chromosome junction region on mouse chromosome 10 (MMU 10). MMU 10 gene order is used as the reference. Distal HSA 21 shows perfect conserved linkage with MMU 10 but HSA 22 is rearranged when compared with homologous regions of the mouse. Immunoglobulin lambda-related sequences shown as shaded blocks are found at the border of each of the rearranged blocks on HSA 22.TIMP3 is near SLC5A4 at 22q12.3 but its mouse ortholog has been mapped several centimorgans distal toCol6a1, on the far side of the HSA 21 conserved sequence block, based on the Jackson Laboratory BSS backcross (Rowe et al. 1994). The vertical line adjacent to the MMU 10 map indicates the region for which a P1 artificial chromosome-based physical map has been established (Wiltshire et al. 1999); the heavy bar locates the 275-kb genomic sequence described here.