Exons 2 through 4 were replaced with a cDNA of the gene with nucleotide substitutions that result in an amino acid substitution of valine for aspartic acid at position 842 (D842V) preceded by a floxed neo-STOP cassette. This mutation is found in human patients with gastrointestinal stromal tumors and leads to a constitutively active form by interfering with the inactive conformation of the ATP-binding pocket. Removal of the floxed neo-STOP cassette is necessary to achieve expression.