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SAN FRANCISCO (AP) -- They're being bred now by the millions, the mutants, created to carry the ghastliest of diseases for the benefit of the human race.

Since researchers published the mouse's entire genetic makeup in map form three years ago, increasingly exotic rodents are being created with relative ease.

There's the Schwarzenegger mouse -- injected with muscle-building genes. The marathon mouse, which never seems to tire. Researchers recently engineered some mice to be extremely addicted to nicotine, and others to be immune to scrapie, a close cousin to the brain-wasting mad cow disease. And scientists are in hot pursuit of a Methuselah mouse, able to cheat death long after its natural brethren meet their maker.

Millions of these and other mutant mice are routinely created now, by injecting disease-causing genes or "knocking out" genes in mouse embryos. Their decreasing cost and increasing availability is helping researchers in pursuit of all manner of disease cures.

Top researchers in the Parkinson's disease field, for example, were more excited by the dopamine-free "knock-out" mouse that Duke University researchers invented than the actual...