Eleven human PSG genes are clustered on chromosome 19q13.1-13.3 [4] . The human PSGs are comprised of a leader peptide followed by one N-terminal immunoglobulin (Ig) variable region-like domain (N-domain) and two or three Ig constant region-like domains [5] . PSG homologues have been identified in species with hemochorial placentation, including nonhuman primates, rats, and mice, all of which are species having several PSG genes that result in expression of several PSG proteins because of alternative splicing [6] . The function of human and murine PSGs is believed to be that of immunomodulators, as one of many reported mechanisms that prevent rejection of the allogeneic fetus by the maternal immune system. This proposed role of PSGs has been suggested by several investigators and is supported by different experimental findings, including the ability of PSGs to induce transforming growth factor beta 1 (TGFB1) [2, [7] [8] [9] [10] .