Major histocompatibility class I proteins display viral and self antigens to potentially responsive cells and are important for the maturation of T cells; β₂-microglobulin (β₂M) is required for their normal expression. Mouse chimeras derived from embryonic stem cells with a disrupted β₂M gene transmitted the inactivated gene to their progeny. Animals homozygous for the mutated β₂M gene were obtained at expected frequencies after further breeding. The homozygotes appeared normal, although no class I antigens could be detected on their cells and the animals are grossly deficient in CD4^-CD8⁺ T cells, which normally mediate cytotoxic T cell function.