KRAS and neoplasm: Interestingly, a retrospective analysis of the NCIC CTG CO.17, which compared cetuximab with BSC, also reported that cetuximab significantly improved PFS in patients with KRAS wild-type left-sided tumors (median: 5.4 vs. 1.8 months), but not in those with right-sided tumors (median: 1.9 vs. 1.9 months).2 Left-sided tumor is derived from the embryonic hindgut, whereas a right-sided tumor is derived from the embryonic midgut.