In order to identify genes that can serve as indicators for predicting the effectiveness of drug treatments in cancers and provide novel methods for predicting the effectiveness of treatments with drugs targeting said genes, transcriptome sequencing was performed of diffuse-type gastric cancer. As a result, in-frame fusion transcripts between the CEP55 gene and the RET gene were identified. It was also found that said gene fusions induce activation of RET protein, thereby causing canceration of cells. Further, it was demonstrated that the RET protein activation and canceration caused by said gene fusion can be suppressed by using a RET tyrosine kinase inhibitor, and that treatments with a RET tyrosine kinase inhibitor are effective in patients with detection of said gene fusion.
