The KRAS mutation is found in several cancers such as pancreatic cancer, colorectal cancer, lung cancer, bile duct cancer and thyroid cancer. The existence of KRAS mutations is often associated with a prognostic marker of drug response. For example, the 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.