We scanned KRAS exon 2/3/4, NRAS exon 2/3/4 and the overlapping epitopes of the EGFR antibodies cetuximab and panitumumab for mutations in pre- and post-treatment tumor tissue of 21 patients with gastrointestinal cancer treated with chemotherapy +/− EGFR antibodies by next-generation sequencing (“tumor tissue” cohort). The gene discussed is KRAS; the disease is neoplasm.