Scientists have, for the first time, discovered how X-rays and radioactive particles cause cancer in humans by damaging DNA. Researchers from the Wellcome Trust Sanger Institute in the UK have been able to identify in human cancers two characteristic patterns of DNA damage caused by ionising radiation, such as gamma rays, X-rays and radioactive particles. these fingerprint patterns may now enable doctors to identify which tumours have been caused by radiation, and investigate if they should be treated differently. Previous work on cancer had shown that DNA damage often leaves a molecular fingerprint, known as a mutational signature, on the genome of a cancer cell. Researchers looked for mutational signatures in 12 patients with secondary radiation-associated tumours, comparing these with 319 that had not been exposed to radiation. “To find out how radiation could cause cancer, we studied the genomes of cancers caused by radiation in comparison to tumours that arose spontaneously,” said Peter Campbell from the Wellcome Trust Sanger Institute who led the study. “By comparing the DNA sequences we found two mutational signatures for radiation damage that were independent of cancer type,” said Campbell. “We then checked the findings with prostate cancers that had or had not been exposed to radiation, and found the same two signatures again. These mutational signatures help us explain how high-energy radiation damages DNA,” he said. One mutational signature is a deletion where small numbers of DNA bases are cut out. The second is called a balanced inversion, where the DNA is cut in two places, the middle piece spins round, and is joined back again in the opposite orientation.