A targeting vector was designed to replace exons 3-7 with a lacZ-neo cassette. The deleted sequence encoded residues 32-298, including the region responsible for interaction with MEF2. RT-PCR and sequencing revealed that exon 2 was spliced to the 5' end of exon 3, which was spliced to the lacZ coding sequence. Transcripts of the wild-type were not found in hearts from mutants. Staining indicated that lacZ expression reflected that of the endogenous gene during pre- and postnatal development. (J:93021)