To identify mutations that can serve as indicators for predicting the effectiveness of drug treatments in cancers such as gastric cancer to provide a means for detecting said mutations and to provide a means for identifying, based on said mutations, patients with cancer or subjects with a risk of cancer, in whom drugs targeting genes having said mutations or proteins encoded by said genes show a therapeutic effect.A method for detecting a gene fusion serving as a responsible mutation (driver mutation) for cancer, the method comprising the step of detecting any one of an ATG3-EPHB1 fusion polynucleotide, a TNIK-RNF123 fusion polynucleotide, or an SLC12A2-NRG2 fusion polynucleotide, or a polypeptide encoded thereby, in an isolated sample from a subject.
