Exon 3 was replaced with a floxed neo/STOP cassette and a modified exon 3 in which nucleotide substitution (GAA to AAA) results in the amino acid substitution of lysine for glutamic acid at position 76 (E76K), mimicking a mutation found in some childhood acute leukemia and juvenile myelomonocytic leukemia (JMML) patients. Cre-mediated recombination removed the floxed stop cassette. (J:332221)