Deletion of all Myf5 coding sequences in exon 1 and replacement with the complete Myog coding region followed by a stop codon. A lox-P flanked neomycin cassette was also inserted downstream of the Myog coding region, but was removed via Cre-mediated recombination in ES cells prior to the generation of chimeric founder mice. This allele expresses a functional Myogenin protein under the control of the Myf5 promoter.