Homozygous mutant ES cells were injected into C57BL/6 blastocysts to form chimeric animals. Mutant ES cells were shown to contribute to most tissues of surviving animals except heart. Embryos with more than 30-40% contribution of mutant cells to the heart exhibit the phenotype of the homozygous mutant. Embryos with a 20-30% contribution of mutant ES cells to the heart exhibited milder phenotypic defects than the homozygous mutant. Embryos with less than 15% contribution of mutant cells to the heart exhibit no significant heart defects at E10.5(J:52597).