Genetic definition of a further gene region and identification of at least three different histocompatibility genes in the rat major histocompatibility system.

Two new recombinant haplotypes of the rat major histocompatibility system, RT1, have been detected in [LEW.1A (RT1a) x LEW.1W (RT1U)] x LEW.1N(RT1n) segregating hybrids. Recombinant r3 carries the RT1.A region (determining classical transplantation antigens) and the RT1.B region (determining strong mixed lymphocyte reactivity and genetic control of antipolypeptide immune responsiveness) of the RT1a parent, bur rejects RT1a skin grafts. Recombinant r4 carries the A and B regions of the RT1u parent, but rejects RT1u skin grafts. The two histocompatibility genes detected are allelic to each other. The relevant locus, designated as H-C, maps to the B-region side of the RT1 system and appears to mark a third RT1 gene region, RT1.C. Availability of haplotypes r3 and r4 allowed the definition of a histocompatibility locus in the B region, H-B. The products of H-C, H-B and of the previously described H-A gene vary in antigenic strength.