A 5kb genomic fragment at the 5' end of the gene was replaced by a neomycin resistance cassette. These deleted sequences include coding regions for the C-terminal portion of the DSL domain. RT-PCR analysis demonstrated that no transcript was detectable in RNA isolated from E10 homozygous embryos, and Western blot analysis on extracts of E10.5 homozygous embryos confirmed that the protein was not expressed.