Figure 4. A Model of the Evolution of the Mammalian MHC.

Organization of the MHC in the mammalian ancestor is similar to that of non-mammals. Class I and II regions are adjacent and the antigen processing genes are found within Class I. Class Ib genes are located outside the MHC but on the same chromosome. The framework and extended regions have assembled. The framework gene order is conserved in both the opossum and eutherians. After the divergence of the eutherian lineage, Class I genes relocated to the framework region.

MYA, million years ago.