Scientists have discovered that a set of proteins that are a part of the body`s natural defences cause mutations in human DNA, which could potentially lead to cancer, says a study.

According to a study led by researchers at the National Institutes of Health, these naturally occurring mutations are just as powerful as known cancer-causing agents in producing tumours, reports Science Daily.

The proteins are part of a group called apolipoprotein B mRNA like (APOBEC) cytidine deaminases. The investigators found that APOBEC mutations can outnumber all other mutations in some cancers, accounting for over two-thirds in some bladder, cervical, breast, head and neck, and lung tumours.

The scientists published their findings online July 14 in the journal Nature Genetics. Dmitry Gordenin, Ph.D., is corresponding author of the paper and a senior associate scientist at the National Institute of Environmental Health Sciences (NIEHS), part of NIH.

He said scientists knew the main functions of APOBEC cytosine deaminases were to inactivate viruses that attack the body and prevent ancient viruses present in the human genome from moving around and causing disrupting mutations.

Because they are so important to normal physiology, he and his collaborators were surprised to find a dark side to them; that of mutating human chromosomal DNA.