In the genome of tumour cells of 11 patients, the scientists observed the insertion of a viral DNA segment from adeno-associated virustype 2, known as AAV2. This virus has been considered non-pathogenic for humans until now. In order to confirm the virus's involvement in the cancer, the research team compared tumour tissues with normal tissues. They thus confirmed their hypothesis: the integration of viral DNA was found more often in tumour cells than in healthy cells in these 11 patients. Moreover, 8 of these patients did not have cirrhosis, and 6 of them presented no known risk factors for liver cancer.