Circulating tumor DNA (ctDNA) is cell-free DNA (cfDNA) that is released from dying/dead cancer cells into the blood stream. It is a biomarker of cancer and is expected to have wide applications, such as the early detection of cancer and monitoring of drug resistance . However, some characteristics of ctDNA make it poorly suited for use in diagnostic applications. One milliliter of blood contains cfDNA from one to several thousand genomes as fragments with an average size of 170 base pairs. The rare mutations from cancer cells must be detected among the vast amount of DNA from normal cells and quantitated.