A team of geneticists reports on page 774 some of the first hard evidence that the cellular power plants called mitochondria deteriorate as people age. The team found that mutations in the 16,500-base mitochondrial genome accumulate with time and in a particularly important region: a 1000-base segment that controls the genome9s replication. Although some scientists are not convinced that mitochondrial changes bring about aging by themselves, the new results have researchers on aging taking a fresh look at the mitochondria.