For 10 patients with pancreatic cancer, both plasma and tumor samples were available. Their variants identified with NOIR-SS are shown in Table 2. Among the 35 variants identified, six were only detected in plasma samples, and all six of these variants were removed by the CV78 filter. To examine whether these variants were derived from leucocytes, we sequenced the corresponding target regions of leucocyte DNA. As shown in S4 Table, no variants were found in leucocytes. As variants in cfDNA are derived from dead/dying cells, it may be difficult to deduce those sequences from living cells. The CV78 filter also discriminated variants present in normal cells from those present in tumor cells. However, it should be noted that six variants present in the tumor samples were also removed by the CV78 filter, indicating the possible loss of true mutations. Except for variants identified in the sample from patient P188, the frequencies of all variant alleles were low. This indicates that these variants do not represent genetic abnormalities present in the majority of the cell population.