D155A, P160A, and K211A mutants also exhibited decreased incorporation levels into MoMLV pseudotypes. However, as the cell-entry assay results are in agreement with our results using purified proteins (Fig. 2B and 2C) , we suggest that the observed phenotype is not due to decreased incorporation levels but due to the mutations themselves. MACV GPC mutants N178A and D159A were not incorporated into MoMLV pseudotypes and therefore were unable to transduce either cell line. MACV GPC mutant W147A behaved unexpectedly in that it had no effect on MoMLV transduction efficiency, despite its decreased precipitation of hTfR1 and attachment to the cell surface of MACVpermissive cells (Fig. 2) . We hypothesize that W147A might be misfolded in the context of MACV GP1D but folded correctly in the context of full-length GPC. The two mutants with the most significant effect on cell entry were the R111A mutant and the triple mutant Y122A/D123A/K169A, exhibiting more than 70% reduction in MoMLV transduction efficiency when compared to wt GPC. Finally, the Y122A mutant, while minimally affecting entry into Vero cells, decreased MoMLV entry into HeLa cells. This is in agreement with our assays using purified proteins (Fig. 2) , in which the Y122A mutant bound efficiently to the cell surface of Vero cells (Fig. 2C) yet was unable to immunoprecipitate hTfR1.