The use of aspirin and other nonsteroidal anti-inflammatory drugs significantly reduces the risk for cancer, but no one has been able to explain why. Now researchers have found that these drugs slow the accumulation of a type of DNA change called somatic genome abnormalities, or SGAs, that lead to uncontrolled cell growth. The researchers tracked SGAs with periodic biopsies over an average of almost 12 years. Over all, the use of nonsteroidal anti-inflammatory drugs was associated with a 90 percent reduction in the rate of mutations. We used techniques used to measure mutation rate in viruses like H.I.V. to measure it in humans," said the senior author, Carlo C. Maley, director of the Center for Evolution and Cancer at the University of California, San Francisco. "We measured whole pieces of chromosomes that are getting deleted or copied." Apparently aspirin slows that rate of mutation. The study, published last month in the journal PLoS Genetics, is very small, Dr. Maley said, and has yet to be reproduced in a larger population. But since most cancers take decades to develop, he added, "if you could just slow it down, you could slow it enough to have people die of something else."