Retroviruses use distinct mechanisms for membrane fusion and cell entry, the mechanisms of which are still poorly defined. In this report, we provided evidence that XMRV entry does not require a low pH or pH-dependent host proteases, but uses a mechanism that is similar to that of typical pH-independent viruses. Interestingly, we find that XMRV entry is enhanced by NH4Cl and BafA1, the two most commonly used agents that neutralize acidic endosomal environments, as well as by leupeptin and cathepsin inhibitor III, which broadly inhibit the lysosomal protease activities. Together, these observations suggest that endocytosis may occur in non-productive entry of XMRV, leading to viral particle degradation. Consistent with this notion, we did not observe specific block of XMRV entry by Dynasore or a dominant negative mutant of Dynamin (K44A) in 293T cells (data not shown). Previous studies have shown that different endocytic pathways mediate entry of some pH-independent retroviruses, including amphotropic and ecotropic MLV as well as HIV-1 [21, 50, 51] , however the exact mechanisms and the underlying significance remain largely unknown. It should be added that, while we have not observed any inhibitory effects of leupeptin and cathepsin III inhibitor on XMRV infection in HTX and 293 cells, we cannot rule out the possibility that cellular proteases may be involved in the XMRV entry of other cell types. In this sense, it is interesting to note that endocytosis and cathepsins were recently shown to affect the entry of several gammaretroviruses, including XMRV, in human TE671 and rat XC cells [52] . Additional studies are warranted to clarify this issue and further characterize the entry pathway of XMRV, perhaps with assistance of the recently developed single molecule labeling and confocal imaging technique.