RB1 and neoplasm: Of the seven matched samples with discordant results, six had unpaired mutations identified in tumor tissue DNA but not in plasma cfDNA in the following genes: TP53 (67%), EGFR (33%), and RB1 (33%); and 2 of 7 paired samples had mutation in plasma cfDNA but not in tumor tissue DNA in TP53 (100%), ERBB2 (50%), and EGFR (50%).