A neomycin resistance cassette replaced a 1kb genomic fragment containing exon 1 sequences encoding the signal sequence and first 107bp of the mature protein. Mutant transcripts were detected by Northern blot and RT-PCR analysis in muscle derived from homozygous mice, but these transcripts do not contain sequences associated with exon 1. FACS analysis on myoblasts isolated from homozygous mice demonstrated an absence of the protein and immunohistochemistry on frozen muscle sections demonstrated that no protein was detectable.