The syncytium induction assay was performed as described previously with some modifications [6, 19] . 293 cells were cotransfected with plasmids encoding XMRV Env or CT truncation mutants plus a GFP-encoding plasmid in order to monitor the transfection efficiency and syncytia formation. Syncytia formation was typically observed and photographed 24 h post-transfection. Where applicable, cells were treated for 5 minutes at 37uC with pre-warmed pH 5.0 buffer (phosphate-buffered saline (PBS), 10 mM MES, 10 m M HEPES) or 0.2-0.5 mM CPZ for 1 min and incubated in normal growth media at 37uC for 1 h.