A hypomorphic allele was generated by the insertion of a full length myogenin cDNA followed by an SV40 T polyadenylation site and a loxP-flanked neomycin cassette into the first exon and deletion of basepairs +48 to +511. Transcript was present at approximately one-fourth the level of controls in muscle. The mutant genomic sequence still contains the transcription start site and the splice donor of the first exon.