The pGT0Lxf gene trap vector, containing a splice acceptor signal followed by betageo (lacZ-neor fusion) cDNA, has integrated into intron 5 of the gene. Immunoblot analysis of homozygous mutant E9.5 whole-embryo lysates detects no FZR1 (CDH1) protein; tissues of heterozygous mutant mice express ~50% of wild-type FZR1 levels. X-gal staining of fixed whole heterozygous E9.5 embryos reveals expression of beta-galactosidase from the gene trap insertion widely throughout the embryo and the placenta.