The cancerous tumour releases DNA (cDNA) into the bloodstream, which is proving to be a biological source of cancer analysis and a diagnostic tool with great potential (detection of residual disease, monitoring of treatment efficacy and identification of resistance mechanisms, monitoring of recurrence and possibly screening). Alain Thierry and his team are among those who have made the greatest contribution to demonstrating the validity and clinical utility of cirDNA. Preventing cancer at any stage by simply analysing drops of blood looks set to become a reality.