Exon 6 was replaced with a neo cassette. The targeted exon encoded amino acids 131 through 161 which make up portions of the kinase catalytic core subdomains VIa and VIb. Northern blot and RT-PCR analyses identified mutant transcript approximately 100 bp shorter than normal transcript. Sequence analysis showed the mutant transcript to contain neither exon 6 nor the neo cassette. Low levels of truncated protein were detected by Western blot analysis, but this protein was reported to lack catalytic activity.