Targeted or gene trap mutations are generated in strain 129SvEv^Brd-derivedembryonic stem (ES) cells. The chimeric mice are bred to C57BL/6J albino mice to generate F1 heterozygous animals. These progeny are intercrossed togenerate F2 wild type, heterozygous, and homozygous mutant progeny. On rare occasions, for example when very few F1 mice are obtained from the chimera,F1 heterozygous mice are crossed to 129SvEv^Brd /C57 hybrid mice to yield additional heterozygous animals for the intercross to generate theF2 mice. Level I phenotypic analysis is performed on mice from this generation