Xenotropic murine leukemia virus-related virus (XMRV) is a gammaretrovirus that was originally identified from human prostate cancer patients and subsequently linked to chronic fatigue syndrome (CFS) [27, 28] . However, recent studies have shown that this virus is a recombinant mouse retrovirus that was likely generated during the passages of a human prostate tumor in nude mice [29, 30] . Moreover, numerous groups have failed to detect XMRV from human prostate cancer samples as well as CFS patients, making the claim of its association with these human diseases questionable [31, 32] . Regardless, it is still important to understand how the Env protein of XMRV mediates membrane fusion and cell entry from the virology perspective, especially in light of the emerging diverse mechanisms of retroviral Env-mediated fusion activation and cell entry [2] . The Env of XMRV shares significant sequence homology with that of other xenotropic and polytropic MLVs (X/P-MLV), especially in the SU subunit, and these viruses share the same xenotropic and polytropic retrovirus receptor 1 (XPR1) for entry [27, 33, 34, 35, 36] . XMRV has been shown to infect a wide range of cell lines derived from different species including humans, with the notable exception of hamster and mouse cells; overexpression of XPR1 in NIH 3T3 and CHO cells renders these cells susceptible to XMRV infection, indicating that XPR1 is the key cellular receptor for XMRV [37, 38, 39, 40, 41] . In this study, we aimed to understand the mechanisms of membrane fusion and cell entry mediated by the XMRV Env protein, particularly the possible role of its relatively long CT (compared to Mo-MLV) and of the viral receptor, XPR1, in modulating this process.