The genomes of particular nonhuman organisms such as the mouse have been studied for a number of reasons including the need to improve sequencing and analysis techniques. These nonhuman genomes also provide powerful sets of data against which to compare the human genome.
Almost every human gene has a counterpart in the mouse, with similar DNA sequences and basic functions. If the 23 pairs of human chromosomes were broken into smaller blocks, those pieces could be reassembled to produce a serviceable model of the mouse genome.
The mouse has long been a favorite for biomedical research, including serving as a premiere model organism in genetics.