Subtitles section Play video Print subtitles Inside a tumour, cells are constantly growing, multiplying and dying. And when cancer cells die, they leave behind clues that are transforming scientists' understanding of the disease. Dying cancer cells break up into microscopic bubbles, which can contain all sorts of molecules - including chunks of the cell's DNA. When cancer cells die close enough to a tumour's blood supply, these DNA-filled bubbles can enter the bloodstream. And as they break down, the tumour DNA is released. This means it can be isolated from just a simple sample of blood. And researchers are now using the DNA in patients' blood samples to learn more about cancer. That's because they reveal a snapshot of the different genetic faults fuelling a tumour's growth, and how they change as it responds to treatment. Scientist want to use this to develop so-called 'liquid biopsies' to help doctors monitor how a patient's tumour is responding to treatment. For example If a blood sample shows signs of tumour DNA after treatment it could mean the patient needs a scan earlier than planned. Or, if new faults are spotted in the DNA, this could help doctors rapidly switch to treatments tailored to these changes. Ultimately, this could open up new ways to personalise each patient's care - and help their doctors stay one step ahead of the disease.