Part of exon 1 and exon 2 were replaced with a lacZ-neo cassette via homologous recombination resulting in the in frame fusion of the first 32 amino acids with lacZ and deletion of part of the kinase domain. Northern blot analysis of homozygous mutant animals did not detect either full length or truncated transcripts. Western blot analysis of mouse embryonic fibroblasts cultured from homozygous mutants did not detect either isoform (46 kDa, 55kDa) of protein product.