Active MHC class Ib genes in rat are pseudogenes in the mouse.

The most telomeric class I region of the MHC in rat and mouse is the M region, which contains about 20 class I genes or gene fragments. The central part carries three class I genes- M4, M5, and M6-which are orthologous between the two species. M4 and M6 are pseudogenes in the mouse but transcribed, intact genes in the rat. To analyze the pseudogene status for the mouse genes in more detail, we have sequenced the respective exons in multiple representative haplotypes. The stop codons are conserved in all mouse strains analyzed, and, consistent with the pseudogene status, all strains show additional insertions and deletions, taking the genes further away from functionality. Thus, M4 and M6 indeed have a split status. They are silent in the mouse but intact in the closely related rodent, the rat.