Acetaldehyde, a metabolic product of alcohol, is suspected to promote cancer. Typically the liver eliminates 99% of acetaldehyde produced. However, liver disease and certain genetic enzyme deficiencies result in high acetaldehyde levels. Heavy drinkers who are exposed to high acetaldehyde levels due to a genetic defect in alcohol dehydrogenase have been found to be at greater risk of developing cancers of the upper gastrointestinal tract and liver. A review in 2007 found "convincing evidence that acetaldehyde... is responsible for the carcinogenic effect of ethanol... owing to its multiple mutagenic effects on DNA." Acetaldehyde can react with DNA to create DNA adducts including the Cr-Pdg adduct. This Cr-PdG adduct "is likely to play a central role in the mechanism of alcoholic beverage related carcinogenesis."