These are changes that occur in the genome of cancer cells. Somatic mutations can be caused by environmental factors. For example, damaging UV rays from the sun cause changes of the C base in DNA to a T this can lead to tumour formation. These can be detected by whole genome sequencing, by comparing the sequence from the patient’s blood sample and the sequence from a sample of their tumour. We can also detect changes to the cancer genome caused by a range of other environmental factors, such as smoking, and viral infections, or the random changes that occur when a cancer is rapidly expanding.