Sequence encompassing 551 nt of the proximal promoter and exon 1 through the majority of exon 3 was replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encoded 75 residues composing the amino terminal, basic region, and most of the helix-loop-helix domain. Transcript was undetected by RT-PCR analysis of cardiac ventricles obtained from homozygous mutant embryos. (J:80681)