Demonstration of a new genetic locus in the major histocompatibility system of the rat.

A new recombination within the major histocompatibility complex (RT1) of the rat has been detected. The recombination occurred between a wild-derived haplotype, provisionally designated p1, and the RT1 haplotype of the BN strain. The recombinant haplotype, designated p3, carries the RT1. A locus (classical histocompatibility antigens) of the BN strain, a locus from the BN strain that codes for the expression of an Ia antigen and strong mixed lymphocyte response (MLR), and a second locus derived from the p1 haplotype that controls the expression of a second Ia antigen, the ability to elicit a strong MLR and the immune response to poly(Glu52Lys33Tyr15). This recombinant therefore demonstrates the division of the RT1.B region into two loci, tentatively designated RT1.B and RT1.D, and provides evidence for the existence of at least four loci in the MHC of the rat.