Hybrid resistance was later linked to the ability of NK cells to react to missing self65, but for years this process was not thought to be involved in solid tissue rejection because Snell’s third “law of transplantation” stated that “skin grafts from either inbred parent strain to the F1 hybrid succeed”66, and MHC homozygous embryonic stem cell-derived teratomas form and persist in MHC heterozygous mice67. This evidence concerns the gene HLA-C and teratoma.