The MoMLV retroviral pseudotypes encoding the alkaline phosphatase (AP) were produced by transfection of 293/GP-LAPSN cells with plasmid DNA encoding individual XMRV Env, CT truncations, or JSRV Env. The MoMLV retroviral pseudotypes encoding the green fluorescent protein (GFP) were generated by co-transfection of 293T cells with pCMV-gag-pol-MLV, pCMV-GFP-MLV (both vectors are kind gifts of François-Loïc Cosset) and plasmids encoding XMRV Env, XMRV Env CT truncations, Ebola GP (pCIneo-Ebola GP) [18] , VSV-G (pMD.G), or MLV 10A1 Env (pCIneo-10A1) [18] . The HIV-1 lentiviral pseudotypes encoding AP were produced by co-transfecting 293T cells with pCMV-HIVD8.2, pHR'CMVAP [66] and plasmid DNA encoding individual Envs. All pseudotypes were harvested 48 and 72 h post-transfection and cell debris were removed by centrifugation at 2,5006 g. MLV pseudovirions were purified by ultracentrifugation on a 20% sucrose cushion for 2 h at 185,0006 g and 4uC, and Western blot was performed to examine SU incorporation using an anti-FLAG antibody. All viral infections were carried out in the presence of 5 mg/ml polybrene (Sigma) and viral titers were determined by AP staining or flow cytometry analysis to measure GFP + cells 48-72 h post-infection. For infection in the presence of drugs or soluble XMRV SU or JSRV SU, cells were first pre-treated with the indicated concentrations of drugs at 37uC or the soluble proteins at 4uC for 1 h, and then incubated with retroviral pseudotypes for 6 h in the presence of drugs or fusion proteins before inactivation using citrate buffer (40 mM sodium citrate, 10 mM KCl, 135 mM NaCl, pH 3.15).