A gene encoding a novel protein that works as a guanine nucleotide exchange factor (GEF) for a Rho family protein being one group of small GTP-binding proteins, namely, a polynucleotide shown by the nucleotide sequence set forth in SEQ ID NO: 1, SEQ ID NO: 3, or SEQ ID NO: 5, or the complementary strand, the equivalents of the polynucleotide, a protein encoded by the polynucleotide, a vector containing the polynucleotide, a transformant containing the vector, an antibody against the protein encoded by the polynucleotide, a method of identifying a compound that inhibits the function of the protein encoded by the polynucleotide and/or the expression of the polynucleotide, a method of determining a disease, a pharmaceutical composition, and a reagent kit are provided.