Previous studies from our laboratory showed that the Ndomain of PSGs is sufficient for their activity in target cells [9, 17] . The N-terminal domain of human PSG1 contains a Gly-Asp-Asp (GDD) motif on a solvent-exposed loop [18] in a region believed to be important for receptor binding and activity [19, 20] . The studies reported here indicate that amino acids G93 and D95, corresponding to the first and third amino acids in this potential receptor binding region, respectively, are not essential for the PSG1 functions we have described, whereas mutations in the salt bridge of the N-domain and elimination of two potential N-linked glycosylation sites render a protein without activity.