“…These include circulating tumor DNA (ctDNA) 5,6 and, to a lesser extent, exosomes. 7,8 In patients with cancer, 1%-10% of circulating or cell-free DNA derives from tumor cells, 9 so the challenge has been to design assays that are sufficiently sensitive to detect low levels of rare mutant ctDNA among the much higher background levels of wild-type ctDNA and total cell-free DNA.…”