CEACAM1-related genes can be subdivided into the PSG and the CEACAM subgroup genes. The birth of the ancestral PSG has been estimated to have occurred some 90 Myr ago [29] , approximately the time of rodent-primate divergence [30] . The rapid, independent expansion of the human and mouse PSG gene families occurred through further gene duplication and exon shuffling events [31] [32] [33] . Surprisingly, we did not find any PSG gene in the dog genome indicating that the PSG function exerted in primates and rodents is dispensable for the dog, possibly because dogs have an endotheliochorial placenta type which is different from the hemochorial placentae of primates and rodents [34] . Semi-allogeneic fetal trophoblast