It’s no secret that heavy drinking is linked to an increased risk of developing cancer, but it’s never really been clear how boozing really does its damage. In a groundbreaking new study published Wednesday, however, scientists report that some cancers occur because alcohol can inflict damage on the DNA of stem cells, which in turn may initiate the development of cancerous tumors.

In the Nature paper, researchers from the University of Cambridge, the Wellcome Trust Sanger Institute, and the Medical Research Council Laboratory of Molecular Biology explain that the damage comes down to the effects of acetaldehyde, a chemical compound and toxin that’s released when the body breaks down alcohol.

Acetaldehyde, the researchers discovered, causes permanent damage to DNA because it can delete and break strands and rearrange chromosomes when the cell lacks a protective enzyme called aldehyde dehydrogenase 2 (ALDH2). Approximately eight percent of the world’s population has an inherited deficiency in this enzyme.

“Our study highlights that not being able to process alcohol effectively can lead to an even higher risk of alcohol-related DNA damage and therefore certain cancers,” co-author Ketan Patel, Ph.D., of the MRC Laboratory of Molecular Biology, told the Guardian.

“But it’s important to remember that alcohol clearance and DNA repair systems are not perfect and alcohol can still cause cancer in different ways, even when people whose defence mechanisms are intact.”

To study this, the researchers genetically engineered mice to have stem cells that didn’t produce ALDH2, the protective enzyme. Then, they gave those mice alcohol. After the mice drank for ten days, the genomes of their stem cells were sequenced, and the results showed that mice lacking the ALDH2 enzyme had four times the damage to their DNA than mice with functioning enzymes. This damage to the stem cells destroyed their ability to create fresh blood cells, which is their major function in the body, the researchers explain, and it may lead to the development of further mutations.

“Our work definitively shows that external factors, like drinking alcohol, can damage DNA in blood stem cells, meaning it could also damage DNA in other types of stem cells,” Patel told Cancer Research UK, a research charity that provided funding for the study. “While we didn’t look at whether these mice got cancer or not, previous studies have show that the type of DNA damage we saw in these mice can considerably increase the risk of cancer.”

A build-up of acetaldehyde can permanently damage DNA.

What type of cancer this process can cause, however, remains to be seen. While this discovery shows alcohol can damage DNA in blood stem cells, Patel notes that there’s no evidence that drinking also means an increased chance of developing blood cancer. Previous research has, however, shown that drinking can increase the risk of developing cancers of the mouth, breast, bowel, throat, and the esophagus.