CD98 is one of the important molecules for development, cell differentiation, cell proliferation, and regulation of cellular function. In this study, CD98 heavy chain (HC) knockout mice were produced and analyzed. Five targeted ES clones were obtained and colony frequency was about 2%. One (clone 113) of the five heterozygous ES cell clones had undergone aberrant recombination at the 5^′ side. The aberrant recombination happened at the site between second intron and 5^′ arm. All lines from correctly targeted clones could not transmit the mutated allele to spermatozoa. The mutated allele derived from the aberrant targeted clone was transmitted to the progeny. However, none of the F2 mice was homozygous for the CD98 mutation, indicating that the targeted disruption of the CD98 gene results in embryonic lethality. The point of embryonic lethality is considered to be between 3.5 and 9.5dps. These findings indicate that CD98 molecules are essential for mouse embryogenesis.