We have used purified MACV GP1 and MoMLV pseudotyped with the MACV glycoproteins in all experiments. Several studies suggest that retroviral vectors pseudotyped with the glycoproteins of arenaviruses adopt the receptor-binding characteristics of the corresponding viruses [17, 20, 27] . Therefore, we believe this surrogate system to be valuable for initial studies of viral cell binding and entry. However, it is important that the system is simplistic and uses only a few viral proteins in the absence of other viral-encoded proteins or genomic elements. Therefore, any results obtained with this system should be verified with infectious virus. Unfortunately, no reverse genetics system has been developed for MACV to date, making it impossible to evaluate the effect of the mutated residues on entry of infectious MACV.