The inherent molecular heterogeneity of metastatic tumors and the ability of cancer genomes to dynamically evolve are not properly captured by tissue specimens. Analysis of cell-free DNA and circulating tumor cells has the potential to change clinical practice by exploiting blood rather than tissue as a source of information. Liquid biopsies are already used to monitor disease response and track the emergence of drug resistance. The suitability of blood-based molecular profiles for early detection and monitoring minimal residual disease is being evaluated. In this review, we address open questions in this fast-evolving field of research.
