It is intended to reveal a polynucleotide serving as a novel causative gene of a cancer and, on the basis of this finding, to provide a method for detecting the polynucleotide or a polypeptide encoded thereby, a kit and a primer set for the detection, a method for screening for a substance that inhibits the polypeptide, and a pharmaceutical composition for the treatment of a cancer, containing the inhibiting substance. The detection method of the present invention detects a BRAF fusion protein or a fusion gene encoding the fusion protein, or a PXN or GMDS fusion protein or a fusion gene encoding the fusion protein in a digestive organ-derived sample obtained from a subject.