Targeted or gene trap mutations are generated in strain129SvEvBrd-derived embryonic stem (ES) cells. The chimeric mice are bred to C57BL/6J albino mice to generate F1 female heterozygous animals.These progeny are crossed to hybrid 129SvEvBrd / C57 F1 mice, derived from crossing 129SvEvBrd mice to C57BL/6J mice, to generateF1A wild-type, female heterozygous, and male hemizygous mice. Level I phenotypic analysis is performed on mice from this generation.