KRAS and colorectal cancer: In addition, a subgroup of 14.7%–17.4% of patients with KRAS exon 2 wild-type mCRC may harbor significant mutations at other KRAS exons (exon 3 at codon 61, and exon 4 at codons 117 and 146) and/or in the RAS isoform NRAS at the same codons assessed in KRAS. Mutations on BRAF have been found in 8%–15% of colorectal cancer and they are mutually exclusive with KRAS mutations.