An FRT-flanked neo cassette with a 5' loxP site was inserted upstream of exon 3. An additional loxP site was inserted downstream of exon 6. Cre-mediated recombination removed exons 3 through 6 and the neo cassette. The absence of transcripts containing exons 4 through 6 and 90% reduction in transcripts with exons 1 and 2 were confirmed by quantitative RT-PCR analysis on E12.5 heart extracts.