BRAF and malignant colon neoplasm: KRAS is very frequently mutated in colon cancer, its mutation being detected in about 40% of cases; KRAS mutations are mutually exclusive of NRAS and BRAF mutations; in more than 90% of these cases KRAS mutations occur at the level of codons 12 and 13; more rarely, these mutations involve exon 3 and exon 4.