A loxP site was inserted upstream of exon 1 and a PGK-neo cassette and loxP site were inserted downstream of exon 1 via homologous recombination. Cre mediated recombination removed exon 1 and the neo cassette. An alternative transcript expressed from exon 1b is detected in the brains, but not the hearts, of homozygous mice. (J:243471, J:268937)