To identify a mutation that can serve as an indicator for predicting the effectiveness of drug treatment in cancers such as leukemia to provide a means for detecting said mutation and to provide a means for identifying, based on said mutation, patients with cancer or subjects with a risk of cancer, in whom a drug targeting a gene having said mutation or a protein encoded by said gene shows 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 an ATF7IP-PDGFRB fusion polynucleotide or a polypeptide encoded thereby, in an isolated sample from a subject.
