EGFR and neoplasm: While cfDNA constitutes a small fraction of the total cell DNA, the absence of EGFR exon 20 p.T790M detection in tissue biopsies may reflect intratumoral heterogeneity, in comparison to liquid biopsy from the same patient, where the shedding of tumor DNA may account for the higher numbers of EGFR exon 20 p.T790M mutations detected.