Expression of the paired receptors CEACAM1 and CEACAM28 in stimulated T cells forward scatter sideward scatter cells are less invasive and concomitantly less prone to encounter the maternal immune system in endotheliochorial placentae in comparison to the hemochorial placentae where the trophoblast cells are bathed in maternal blood [35] . Therefore, PSG could be involved in the regulation of trophoblast invasion and/or modulation of the maternal immune system as has been suggest before [3, 36] . Alternatively, the function of the PSG could have been taken over by an unrelated gene family due to functional convergence at the molecular level, similar to that found for natural killer (NK) cell receptors, where killer cell immunoglobulin-like receptors (KIR) and Ctype lectin-like receptors (Ly49) are used in primates and rodents, respectively.