The PSG concentration increases progressively to reach a plateau in the last 4 wk of pregnancy. At 200 lg/ml, PSGs are the most abundant fetal proteins in the maternal bloodstream during late pregnancy [62] . At this time, and to our knowledge, no specific antibodies can distinguish the different members of the human PSG family, but splice variants for most of them have been reported to be expressed, with most of these including the N-domain [19] . Recently, transcripts for PSG1 were found in sperm, and PSG1 protein was detected during zygotic development, suggesting a possible role for this protein in the early stages of embryogenesis and/or implantation [63] . Within the N-domain, most PSGs have an RGD or an RGDlike sequence. The RGD motif forms the minimal functional binding unit in some integrin ligands [64] , and its presence in one of the solvent-exposed loops of most PSGs was assumed to be required for PSG function [20] . Within this sequence, the conservation of the aspartic acid (D) at position 95 has been cited as essential for function [18, 19] . Our results indicate that at least for some functions of PSGs, the G in position 93 and the D at position 95 are not essential. Binding assays showed that although the PSG1GDD!SDL mutant binds to target cells, such as HTR-8/SVneo, significantly over the control protein, the binding (measured as mean fluorescence intensity) was approximately half that of the wild-type protein. Whether other, as-yet-unknown functions of this protein could be mediated by these amino acids, or how the mutation affects the affinity of the interaction with the receptor, remains to be determined.