EGFR and neoplasm: Furthermore, a patient with NSCLC and an EGFR L858R mutation found in an original tumor biopsy had cfDNA collection after developing secondary resistance to the EGFR tyrosine kinase inhibitor erlotinib, which in addition to a known EGFR L858R mutation (0.11%), revealed an EGFR T790M mutation (0.04%), plausibly explaining secondary resistance to erlotinib.