The targeting construct inserted downstream of exon 2 a loxP site, a duplicate copy of exon 2 and an FRT-flanked neomycin resistance cassette with a 3' loxP site. mRNA analysis confirms the inclusion of a duplicated exon 2 in muscle. Immunofluorescence and immunoblot analysis indicate that expression is essentially absent, although some muscle shows very low-level trace expression. (J:233298)