Exon 49, which encodes the N2B extensible region, was replaced with a FRT-flanked neomycin resistance cassette. Chimeric mice were mated with a Flp expressing deleter strain to remove the neomycin cassette, leaving a single FRT site. The deletion of exon 49 leads to splicing of exon 48 to exon 50, which maintains the ORF and produces a mutant form of the protein that lacks a N2B region. The N2B-deficient protein is integrated properly into sarcomeres as demonstrated by immunofluorescence labeling.