By finding a novel tetrahydrofolate synthetase gene and a protein encoded by said gene, a method for identifying a compound which inhibits cell growth accelerating activity of said protein is provided, and a judging method, a preventing method and a treating method of colon cancer are provided. A DNA comprising a nucleotide sequence of from the 94th to 2934th positions of the nucleotide sequence of SEQ ID NO:1 of the SEQUENCE LISTING; a polynucleotide which specifically hybridizes with said DNA; a protein encoded by said DNA; a recombinant vector comprising said DNA; a transformant comprising said recombinant vector; an antibody for said protein; a method for producing said protein; a method for identifying a compound which inhibits cell growth accelerating activity possessed by said protein; a method for judging colon cancer, characterizing in that expressed amount of said DNA is measured; a kit for judging colon cancer; a preventive agent and/or therapeutic agent for colon cancer.