They Die by the Score

No creatures make more sacrifices for science--albeit involuntarily--than the mice and rats of lab research. Some 30 million are used each year in the U.S. alone.

Reasons: They're small and easily bred, and 99 percent of mouse genes (and likely rat genes too) correspond to human ones. Here, recent rodent-based research.

Fighting AIDS and breast cancer
Investigators at UCSF made a mouse version of HIV, and a U. Wisconsin team created the first "knockout" rat; it was stripped of a gene that curbs human breast cancer.

Extending the human life span
The Methuselah Mouse Prize promises $10 million for rejuvenating an old mouse so that it lives five years instead of three.

Curbing depression
To test new antidepressants, French investigators bred "depressed" mice by selecting animals that don't fight back when subjected to stress, such as being held by their tails.