N-glycans can have high mannose, complex or hybrid structures. The various glycosylation inhibitors target different steps in protein glycosylation and can be used to manipulate the carbohydrate composition of glycoproteins. The inhibitor CST blocks glucose trimming, and DMM and SW inhibit successive steps in mannose trimming. The fact that all of these inhibitors along with the sugar analog 2DG and glycosylation inhibitor Tu relieved the resistance of M. dunni cells to MoMLV and of NIH 3T3 to Spl574 suggests that these viruses are most effectively blocked by the large complex oligosaccharides produced in the terminal stages of glycosylation. These results, taken together, suggest roles for N-glycans in virus entry that are virus-specific and cell-specific, and also indicate that this regulation may be sensitive to small sequence changes in both virus and receptor. These results indicate that N-glycans broadly regulate ecotropic gammaretrovirus interactions with the CAT-1 receptor in cells of their natural host [21] , although it is possible that glycosylated proteins other than CAT-1 may contribute to this resistance.