In the α1/α2 co-domain, residues inside and outside the PBR evolve differently. A) The tree was generated by comparing the 113 residues of the α1/α2 co-domain that fall outside of the antigen recognition site (ROARS). B) The tree was generated by comparing the 67 residues that take part in the PBR. Percentage bootstrapping support values above 60% are generally considered as highly significant. Class Ia molecules appear in red. SLA is a pig class Ia molecule chosen as an outlier, since the split with the ancestor of pigs occurred much earlier than that between primates and rodents . CD94L molecules are shown in blue, and M3 molecules in green.