A set of proteins involved in the body's natural defenses produces a large number of mutations in human DNA, according to a study led by researchers at the National Institutes of Health. The findings suggest that these naturally produced mutations are just as powerful as known cancer-causing agents in producing tumors.

The proteins are part of a group called apolipoprotein B mRNA-editing enzyme catalytic polypeptide-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 tumors.

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 Environmentalsince the release last week of a transcript of a deposition from former IRS official David Fisher. After being subpoenaed, he revealed that some IRS officials, including him, had raised concerns about the legality of the cost sharing reduction payments in 2014.

Fisher said most of the IRS officials were assuaged of their concerns after a meeting with White House officials in January 2014. The Office of Management and Budget distributed a legal memo at the meeting that justified the legality of the payments, Fisher said. The IRS officials were not allowed to take notes or keep copies of the memo, an arrangement that Fisher called “a little unusual.”

Republicans have issued a subpoena to the White House for a copy of the memo, but the White House has declined