A targeting vector replaced the amino-terminal domain with a floxed neomycin and hygromycin resistance cassette upstream of the basic helix-loop-helix domain. The endogenous ATG was mutated to GTG, leaving the first ATG codons suitable for translation located in the chromatin remodeling and basic domains, at residues 69 and 82, respectively. This alleles was used to generate Myf5tm2Tajb.