A new H-2-linked class I gene whose expression depends on a maternally inherited factor

The maternally transmitted antigen (Mta) is expressed on the cells of most strains of mice^1,2. It is a medial histocompatibility antigen ³, that is, it is recognized by unrestricted cytotoxic T lymphocytes as are major H antigens, but unlike these it is a weak transplantation antigen and does not itself restrict the T-cell recognition of minor H antigens. All other medial H antigens are encoded by genes closely linked to the major histocompatibility complex³, H-2 in the mouse. By contrast, Mta appeared to follow extrachromosomal, maternal inheritance. Several substrains of NZB, NZO and non-inbred European NMRI mice are Mta-negative. Females of these strains bear only Mta^- offspring, while females of the inbred Mta⁺ strains bear only Mta⁺ offspring. Repeated backcrossing from Mta⁺ females to NZB or NMRI males has shown that, given the right cytoplasmic genes, the chromosomal genes of these Mta ^- strains permit expression of Mta². As the Mta type of a mouse cannot be influenced by embryo transfer or foster nursing^2,4, we concluded that it was determined by a cytoplasmic factor (Mtf), transmitted through the egg. We now show that a gene, Hmt, closely linked to the H-2 complex, is also required for expression of Mta.