Galectin-1 is an anti-inflammatory lectin with pleiotropic regulatory functions at the crossroads of innate and adaptive immunity. It is expressed in immune privileged sites and is implicated in establishing maternal-fetal immune tolerance, which is essential for successful pregnancy in eutherian mammals. Here, we show conserved placental localization of galectin-1 in primates and its predominant expression in maternal decidua. Phylogenetic footprinting and shadowing unveil conserved cis motifs, including an estrogen responsive element in the 5 promoter of LGALS1, that were gained during the emergence of placental mammals and could account for sex steroid regulation of LGALS1 expression, thus providing additional evidence for the role of galectin-1 in immuneendocrine cross-talk. Maximum parsimony and maximum likelihood analyses of 27 publicly available vertebrate and seven newly sequenced primate LGALS1 coding sequences reveal that intense purifying selection has been acting on residues in the carbohydrate recognition domain and dimerization interface that are involved in immune functions. Parsimony-and codon model-based phylogenetic analysis of coding sequences show that amino acid replacements occurred in early mammalian evolution on key residues, including gain of cysteines, which regulate immune functions by redox status-mediated conformational changes that disable sugar binding and dimerization, and that the acquired immunoregulatory functions of galectin-1 then became highly conserved in eutherian lineages, suggesting the emergence of hormonal and redox regulation of galectin-1 in placental mammals may be implicated in maternal-fetal immune tolerance.decidua ͉ estrogen ͉ glycocode ͉ immune-endocrine cross-talk ͉ pregnancy T he success of mammalian pregnancy, in which the developing fetus and mother exchange nutrients, gases, and other molecules via the chorioallantoic placenta, requires maternal immune tolerance to fetal allo-antigens (1-4). This tolerance presumably prevents the occurrence of exaggerated inflammation at the implantation site and reduces the danger of destructive immune attacks on the fetus, a danger that the first mammals with an invasive placenta would have faced (5, 6). It seems likely that mechanisms for immune tolerance to invasive placentation were already functioning in the early placental mammals and that immunoregulatory molecules, which had existed before the mammalian placenta evolved, were incorporated in this tolerance and have undergone evolutionary modifications coincident with the emergence of the mammalian placenta. Here, we present evidence that such modifications occurred in a key immunoregulatory molecule, galectin-1.Molecules that have been implicated in conferring maternalfetal immune tolerance include galectin-1, B7 proteins, Crry, Fas ligand, HLA-G, indoleamine 2,3-dioxygenase, and killer cell immunoglobulin-like receptors (2-4, 7-11). These proteins are involved in pathways regulating adaptive or innate immune responses at the maternal-fetal interface, and their di...