While it’s always a good idea to quit smoking, new research from the American Heart Association suggests that irreparable damage to the DNA has already been done.

According to new research published in the journal Circulation: Cardiovascular Genetics, smoking leaves a “footprint” on the human genome in the form of DNA methylation, the process in which cells control gene activity. Tracking which genes are turned on or off could provide potential targets for new drug therapies.

“These results are important because methylation, as one of the mechanisms of the regulation of gene expression, affects what genes are turned on, which has implications for the development of smoking-related diseases,” said Stephanie J. London, MD, DrPH, deputy chief of the Epidemiology Branch at the National Institute of Environmental Health Sciences, National Institutes of Health, in a statement. “Equally important is our finding that even after someone stops smoking, we still see the effects of smoking on their DNA.”

Researchers have long since known that long-term smokers are at an increased risk of developing diseases including some cancers, chronic obstructive pulmonary disease and stroke. The medical community remains unsure of the exact molecular mechanisms that cause these diseases, but previous studies have shown a more direct link between DNA methylation and cardiovascular disease.

Researchers conducted a meta-analysis of DNA methylation sites across the human genome using blood samples taken from nearly 16,000 participants from 16 groups of the Cohorts for Heart and Aging Research in Genetic Epidemiology (CHARGE) Consortium, including a group of the Framingham Heart Study that has been followed by researchers since 1971.

The researchers compared DNA methylation sites in current and former smokers to those who never smoked.

Most notably, the researchers found that DNA methylations sites were associated with more than 7,000 genes. While the majority of those methylation sites returned to normal within five years of quitting smoking, some sites persisted 30 years after quitting.

“The encouraging news is that once you stop smoking, the majority of DNA methylation signals return to never smoker levels after five years, which means your body is trying to heal itself of the harmful impacts of tobacco smoking,” said Roby Joehanes, first author and an instructor at Harvard Medical School in Boston.