Four years ago, researchers in the CRUK TRACERx team at the Crick and UCL, alongside an industry-collaborator Natera, developed a personalized test that uses circulating fragments of tumor DNA (ctDNA) in the blood of patients with lung cancer to predict if their cancer would come back after surgery – even before it would be visible on a scan. And last week, a personalized and tumor-informed blood test called Signatera proved effective at defining which patients will benefit from immune therapy after surgery in a landmark clinical trial. Charles Swanton, head of the Cancer Evolution and Genome Instability Laboratory at the Crick and UCL, and Chief Clinician at Cancer Research UK, explains how this technology evolved from an idea into a potentially life-saving clinical test.