The members of the LEW-LEW.C-4A (AUG) congenic pair differ in at least two alloantigenic systems, RT4 and RT6.

In rats, two alloantigenic loci, RT4 and RT6, were separated by recombination in a backcross population of (LEW X LEW.C-4A) X LEW hybrids using cytotoxicity tests with alloantisera and immunofluorescence with monoclonal antibody P4/16 in combination with histogenetic (skin graft) testing. Two new recombinant congenic strains were established, LEW.C-4A/I of the genotype CCRT4aaRT6aa and LEW.6B of the genotype ccRT4bbRT6bb. In the rat, transplantation results localize the histocompatibility locus RT4 closer to the locus albino, the T-cell RT6 locus without any proven histocompatibility effect is closely linked to the haemoglobin locus Hbb, in contrast to the mouse in which the haemoglobin (Hbb) and histocompatibility H-1 loci are closely linked.