Is the H-2K locus of the mouse stronger than the H-2D locus?

Recently, evidence has been gathered showing that the H-2 complex of tlic mouse consists of only two histocompatibility genes (regions), H-2K and H-2D (Klein & Shreffler 1972). The rest of the H-2 regions described in the literature (C,V,E,A) are probably serological artifacts. Kychlikovi and co-workers ( 197 1 ) reported that significant transformation of allogeneic lymphocytes in mixed cultures (MLC) occurred when congenic strain combinations differed at the H-2I< locus and did not occur when the combinations differed at the H-2D locus. According to Dkmant ( 1970), a similar phenomenon can be observed in the graft-versus-host (GVH) reaction. Here again, the 13-2K incompatibilities lead to a strong reaction while H-2D incompatibilities cause only insignificant or very mild splenomegaly. The authors interpret these results as evidence for the “superiority of the K-end incompatibilities over the D-end incompatibilities” (RychlikovA et al. 1971). I n this communication, evidence is presented that : 1 ) skin graft rejection does occur across the H-2D barrier, and 2 ) H-2K, at least in the combinations studied, presents a stronger histocompatibility barrier than H-2D. Four intra-H-2 crossovers (Table I ) , two proven (i.e. H-2h-3Sg,H-2i-2Sg) and two suspected 1i.e. H-2a,H-2m), and four H-2 chromosomes from which the crossovers were derived ( H-2b,H-2d,H-2k,H2 4 ) , were arranged in eight different donor-recipient combinations (Table 11). Four of these combinations differed at the H-2K locus and the other four at the H-2D locus. All the H-2 chromosomes were on the same genetic background of the C57RL/lOSn ( = B l O ) strain, thus ex-