Mice are genetically very similar to humans. They also reproduce rapidly, have short life spans, are inexpensive and easy to handle, and can be genetically manipulated at the molecular level.

In addition to the human genome, the genomes of about 800 organisms have been sequenced in recent years. These include the mouse Mus musculus, the fruitfly Drosophila melanogaster, the worm Caenorhabditis elegans, the bacterium Escherichia coli, the yeast Saccharomyces cerevisiae, the plant Arabidopsis thaliana, and many microbes.

Genome sizes of Mus musculus being studied

organism: Mus musculus (mouse)

estimated size (base pairs): 2.6 billion

estimated gene number: ~25,000

average gene density: 1 gene per 100,000 bases

chromosome number: 40