The majority of bad breath cases are the result of either bacteria buildup in the mouth, or simply having a meal filled with garlic and onions. But for others, bad breath is a chronic condition that may be caused by a genetic mutation, a new study finds.

The study, published in Nature Genetics, is the result of about 25 years of ongoing research. Researchers from Radboud University in The Netherlands and the University of Warwick studied three families – from Germany, Portugal and The Netherlands – who had multiple relatives affected by chronic halitosis.

After examining breath, blood and urine samples from the families, the researchers found that every family member who suffered from halitosis produced a lot of sulfur-based compounds in their breath, particularly methanethiol, which is known to have an unpleasant boiled-cabbage smell. Normally, methanethiol is produced during digestion but is broken down in the body, according to the researchers.

The patients with halitosis also had a mutation in the SELENBP1 gene. The gene produces the protein selenium binding protein 1, which converts methanethiol into other compounds.

The mutation doesn’t allow methanethiol to properly break down, and leads to high levels of the compound in the patients’ blood. As the researchers explained, when blood reaches the lungs, the smelly sulfur compounds are exhaled, causing foul-smelling breath.

To confirm the findings, researchers from UC Davis edited genes in mice to “knockout” the equivalent of the SELENBP1 gene. They found that the “knockout” mice had low levels of selenium binding protein 1 and high levels of methanethiol and other volatile sulfur compounds in their blood as well.

“While we didn’t put our noses up to the mice’s mouths, we did measure high amounts of some of these odor-forming chemicals in their blood, matching precisely what was found in the patients,” said Kent Lloyd, director of the Mouse Biology Program at UC Davis.

The team estimates that about 1 in every 90,000 people carry the SELENBP1 gene mutation, and there currently is no cure for halitosis.

SELENBP1 has also been linked with some cancers in humans, although the association is still not clear.

“It’s important to identify the cause of persistent halitosis, and differentiate that cause from relatively benign causes (e.g., gum disease) and the more morbid causes such as liver cirrhosis,” said Lloyd.