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 biliary tract cancer. As a result, in-frame fusion transcripts between the FGFR2 gene and other gene (BICC1 or AHCYL1 gene) were identified. It was also found that said gene fusions induce activation of FGFR2 protein, thereby causing canceration of cells. Further, it was demonstrated that the FGFR2 protein activation and canceration caused by said gene fusions can be suppressed by using an FGFR2 inhibitor, and that treatments with an FGFR2 inhibitor are effective in patients with detection of said gene fusions.
