The access to tumor material from a simple blood sample is generally called liquid biopsy. In this review, we describe the needs for such a technique, and the potential applications in oncology. We describe different classes of targets of liquid biopsy: circulating tumor cells, circulating tumor DNA, and exosomes, presenting current evidence, clinical meaning and utility of each class. Then, we explore in more depth methods for the characterization of CTC and ctDNA -the two targets actually considered closer to the bedside-, from a genetic, epigenetic and, in case of CTC, proteomic point of view. We compare applications, promises and limitations of CTC and ctDNA in the clinical setting. Finally, we offer a perspective on the clinical evaluation of genetic characterization from liquid biopsy samples, taking into consideration that certain mutations — often considered to be malignant — have been frequently described in healthy subjects.
