Smokers who quit never fully recover from the damage cigarettes have wrought, says a new study. When a smoker quits, risks of heart disease and stroke drop significantly. "You are reducing the risk of disease by quitting," cancer biologist Raj Chari tells Nature. "But it isn't going back to zero." That's because DNA in the tissue of the lungs is forever changed by cigarette smoke. When Chari and his team scraped tissue from the airways of nonsmokers, current smokers, and former smokers, they found that some of the lung-cell DNA that had been mutated by smoking was back to normal in the former smokers. But 124 other genes, some of which have been linked to lung cancer and emphysema, hadn't repaired themselves. These genes are the reason heavy smokers can develop lung disease and cancer even decades after they've quit. "Cells in the airway appear to have changes at a molecular level that persist many years after quitting," says lung specialist Avrum Spira.