The cell-cell fusion activities of XMRV Env and mutants in CHO and CHO/XPR1-expressing cells were then examined. For this purpose, we labeled CHO or CHO/XPR1 cells with CMTMR, and co-cultured them with the effector 293T/GFP cells expressing XMRV Env or truncation mutants plus GFP. We observed that, surprisingly, CT624 and CT613 reproducibly induced a detectable level of cell-cell fusion activity in the nonpermissive CHO cells (p,0.05) (Fig. 6B) , despite the fact that this cell line is nonpermissive for XMRV infection (Table 1) . Interestingly, overexpression of human XPR1 in CHO cells only slightly increased the fusion activities of XMRV Env CT mutants, CT624 and CT613 (Fig. 6B) , despite their significantly increased pseudoviral titers (Table 1) . These results, along with the data using 293/XPR1 cells described above, imply that XPR1 may not be the sole trigger for XMRV Env-mediated membrane fusion and cell entry.