A 2.5 kb region of the gene spanning exons 12-14 were replaced with a PGK-neo cassette via homologous recombination. The gene targeting event results in deletion of the C-terminal SH2 domain and the ATP-binding site of the tyrosine kinase domain (amino acids 497-607). Homozygous mutant embryonic stem cells were identified via Southern blot analysis. (J:76509)