Next, we specifically quantified the tumor-derived fraction of cf-nDNA, termed circulating tumor DNA (ctDNA), using droplet digital PCR (ddPCR) targeting HPV16 E6 and E7 sequences and the NPM1 mutant type A sequence prevalent in the HPV+ HNC and AML patient cohorts, respectively (Supplemental Figure 6, A and B). Here, NPM1 is linked to acute myeloid leukemia.