An in-frame lacZ gene and neomycin selection cassette replaced approximately 8 kb of gene sequences, including exons 1-4 and most of exon 5. Northern blot analysis on RNA derived from muscle of homozygous mice confirmed that no wild-type transcript is expressed from this allele. Beta-galactosidase is expressed from this allele under the control of the endogenous promoter. (J:77660)