A new lymphocyte-activating determinant (Lad) locus expressed on T cells was identified, mapping in the I-C subregions of H-2k and H-2d haplotypes. The mixed lymphocyte reaction stimulation could be inhibited by anti-Ia sera made in strains incompatible for this chromosomal segment. Experiments with purified lymphocyte cell populations suggested that this Lad locus was expressed on T cells. Further, only purified T cells were able to remove the inhibiting activity from the anti-Ia sera. I-C subregion gene(s) seem to code for products selectively expressed on a subpopulation of T cells.

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