HHMI investigator Nancy Bonini's sprawling laboratory at the University of Pennsylvania is filled with fruit flies. But they're not swarming. They're sequestered in glass tubes stoppered with cotton. Endless boxes of the white-tufted tubes crowd the benches and stack high on the shelves, overflowing onto the floor.

Postdoctoral fellow Hyung-Jun Kim taps the bottoms of two tubes on a lab counter, agitating the roughly two dozen flies in each. In one tube, flies zip around and scale the glass, some nearly reaching the top. In the other, the flies are sluggish. Most crawl on the bottom; a few make feeble attempts to climb but don't get far. "They don't look as happy," remarks Bonini.

The source of the stark contrast in energy and climbing ability is surprising: a human gene that is involved in amyotrophic lateral sclerosis (ALS), known as Lou Gehrig's disease. The sluggish flies carry the gene and an abundance of the protein it produces, while the sprightly flies do not.

Illustration: The Heads of State