A PGK-neomycin resistance cassette replaced exon 17, which encodes the amino terminal region of the tyrosine kinase domain. Northern blot analysis of homozygous mutant embryos using a 5' cDNA probe detected a ~3kb mRNA, and detected no 7kb full length transcript using a 3' probe. Immunoblot analysis of lung tissue from homozygous mutant mice detected the endogenous soluble form of the protein, and a novel truncated form that lacks the cytoplasmic region. No full length protein was expressed from this allele.