The current model of EBOV and MARV fusion is that GP 1 cleavage by endosomal proteases removes heavily glycosylated domains, exposing the receptor binding domain on GP 1 and enabling binding to NPC1 [95] . The membrane-bound fusogenic GP 2 undergoes a low pH-dependent rearrangement to an extended conformation resulting in the fusion of virion and endo-lysosomal membranes [96] . In support of the pH-dependence of GP-mediated fusion, pre-treatment of cells with ammonium chloride prevented entry of a MARV GP-pseudotyped virus [183] . A recent report with live MARV showed that ammonium chloride inhibited entry and replication, but that Bafilomycin A 1 , which specifically inhibits vacuolar-type H(+) ATPase and prevents re-acidification of vesicles of the central vacuolar system, surprisingly had no effect [172] . Following viral fusion with the endosomal membrane, the nucleocapsid is released into the cytoplasm (Figure 7 ).