A neomycin resistance cassette replaced 2.7 kb of sequence including 52 bp of the exon that encodes the beginning of the C-terminal cysteine rich region. RT-PCR analysis demonstrated that a mutant transcript carrying the deletion was made in homozygous and heterozygous mice. Western blot analysis on extracts of muscle or lung tissue derived from homozygous mice confirmed that no detectable protein was expressed from this allele.