We next determined the role of XPR1 in XMRV Env-induced membrane fusion by using nonpermissive CHO cells and CHO cells expressing human XPR1 (CHO/XPR1). The CHO/XPR1 cell line was established by transducing CHO cells with a LXSN retroviral vector encoding XPR1 [33] . Expression of XPR1 in the CHO/XPR1 cell line was demonstrated by the specific binding of soluble XMRV SU fusion protein to those cells as shown in Figure 2B , and was further confirmed by immunostaining using an anti-XPR1 antibody (Fig. 6A) . The titers of XMRV wildtype and CT truncation mutants in these two cell lines are shown in Table 1 . CHO cells were apparently not susceptible to XMRV Env pseudoviral infection (Table 1) , consistent with previous reports from other groups [38, 39, 40, 47] . Overexpression of XPR1 in CHO cells resulted in a titer of 10 4 IU/ml for the wildtype and somewhat reduced titers for the CT truncation mutants (Table 1) . Overall, these data support the notion that XPR1 is a critical cellular receptor for XMRV.