Taking a major step toward identifying one cause of aging, researchers have shortened the life of mice and created signs of old age by injecting a small genetic defect in the mice’s mitochondria, the tiny power plants within each cell.

The experiment in Sweden offers the first hard evidence of a decades-old theory that mutations in the mitochondria are one of the causes of age-related illnesses. Earlier research had shown that such defects build up in people as they grow old, but scientists were not sure whether that was a cause or a symptom of aging.

”What we have now is this clear-cut cause-and-effect relationship,” said Dr. Nils-Goran Larsson, a genetics professor at the Karolinska Institute in Stockholm and the senior author of the study. ”It will provide a completely new angle to treat aging-related problems.”

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