A targeting vector was designed to replace exon 3 with a deletion of the 105 bp exon 3 sequence and the intron sequences (15 bp on both sides) that flank exon 3 and a FRT flanked neomycin cassette. The neomycin cassette was removed via Flp-mediated recombination. Western blot analysis of heterozygotes indicates that protein level in hearts is reduced to 58% of wild-type hearts, indicating impaired expression of this allele. (J:215190)