In an accompanying publication we show that a subpopulation of T lymphocytes, which includes allotype suppressor T cells, selectively expresses I-region determinants. In this report, we show that these determinants are controlled by a new locus, Ia-4. Unlike the classically defined Ia antigens, they are not found on B lymphocytes. Antibody against Ia-4 determinants cannot be detected by conventional dye exclusion cytoxicity assays, suggesting that they are present on a small subpopulation (less than 10%) of peripheral T lymphocytes. The Ia-4 locus marks a new I subregion, provisionally designated I-J. This chromosomal segment is defined by the crossover positions in strains B10.A(5R) (K-end boundary) and B10.HTT (D-end boundary), and maps between the I-B and I-C subregions.

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