PROBLEM TO BE SOLVED: To provide, for somatic ErbB3 mutations in cancer, an ErbB3 cancer detection agent, a diagnosis and prognosis method, and a method of treating the cancer.SOLUTION: Provided is a method using an ErbB3 gastrointestinal tract cancer detecting agent, which comprises a polynucleotide represented by a specific formula and includes an agent capable of specifically binding to an ErbB3 mutation in an ErbB3 nucleic acid sequence.SELECTED DRAWING: Figure 4a【課題】癌における体細胞ＥｒｂＢ３変異に関し、ＥｒｂＢ３癌検出薬、診断及び予後診断の方法、並びに、癌を治療する方法の提供。【解決手段】特定式で表わされるポリヌクレオチドを含み、ＥｒｂＢ３核酸配列中のＥｒｂＢ３変異に特異的に結合できる試薬を含む、ＥｒｂＢ３消化管癌検出薬を用いる方法。【選択図】図４ａ
