KRAS mutation is found in several cancers such as pancreatic cancer, colorectal cancer, lung cancer, biliary tract cancer and thyroid cancer. The existence of KRAS mutations is often related with a prognostic marker to drug response. For example, KRAS mutation is considered a strong prognostic marker for drug response of tyrosine kinase inhibitors such as Gefitinib (Iressa) or Erlotinib (Tarceva). Recently, KRAS mutation is often detected in colorectal cancer and may be related with drug response to Cetuximab (Erbitux) or Panitumumab (Vectibix) that is used for colon cancer therapy. Therefore, examination of KRAS mutation is needed to determine drug resistance of patients with colorectal or lung cancers and will be helpful for cancer therapies.