The present invention relates to a novel method for identifying a gene which may be an indicator for predicting the effectiveness of medicine-based cancer treatment and predicting the effectiveness of the treatment targeting the gene. As a result of carrying out overall transcriptome sequencing for pulmonary adenocarcinoma, an in-frame fusion transcription product of a KIF5B gene and an RET gene was identified. KIF5B-RET gene fusion was detected in 6/319 cases (2%) of pulmonary adenocarcinoma in Japanese subjects and in 1/80 cases (1%) of pulmonary adenocarcinoma in American subjects, and it was found that the gene fusion was a causative mutation (driver mutation) in the development of cancer as none of the seven cases showed a well-known activated mutation, i.e. EGFR, KRAS, and ALK oncogenes. The gene fusion is thought to cause constant activation of the RET tyrosine kinase protein, and it was discovered that a treatment using an RET tyrosine kinase protein inhibitor was effective for patients in which the gene fusion has been detected.
