Circulating tumor DNA (ctDNA) is tumor-derived fragmented DNA with an average size of 170bp, mixed with cell free DNA (cfDNA) of other sources in blood circulation. Although the mechanisms of its release have not been fully addressed, apoptosis and/or necrosis of tumor cells and serum exosome are considered as its main source, which makes it a genomic reservoir of different tumor clones. Also, as its half-life is up to hours, ctDNA is reflecting the most up-to-date status of tumor genome. Hence, it allows for noninvasive molecular characterization of tumors,which can be qualitative, quantitative and used for disease monitoring.