The KRAS mutation is found in several cancers such as colorectal, lung, thyroid, pancreatic, cholangiocarcinoma. KRAS mutations are often found in codons 12 and 13 of exon 2 and result in an abnormal change of the p21-ras protein growth signal. This abnormal change related to cell growth and division can cause the process of cancer development by over-delivering the signal. The existence of KRAS mutations is often associated with a prognostic marker of drug response. For example, KRAS mutation is considered a strong prognostic marker for drug response to tyrosine kinase inhibitors such as gefitinib (Iressa) or erlotinib (Tarceva). Recently, KRAS mutation is often detected in colorectal cancer and may be related to drug response to cetuximab (Erbitux) or panitumumab (Vectibix) used for colon cancer therapy. Therefore, KRAS mutation testing is necessary to determine drug resistance in patients with colorectal or lung cancer and will be useful for anticancer therapies.