Powerful new genetic technologies enable clinicians to detect and sequence tiny amounts of free DNA circulating in blood. DNA gets into blood when cells fall apart. Abnormal DNA from diseased cells can be detected, enabling clinicians to detect cancer or monitor tumor growth by liquid biopsy. In this JAMA Clinical Reviews podcast, we talked with Victor E. Velculescu, MD, PhD, from the Johns Hopkins School of Medicine, and JAMA medical writer M.J. Friedrich about this new technology. Related articles: Cancer DNA in the Circulation: The Liquid Biopsy Going With the Flow: The Promise and Challenge of Liquid Biopsies Finding the Rare Pathogenic Variants in a Human Genome