We next performed syncytia-forming assay in 293 cells and assessed the membrane fusion properties of XMRV Env and mutants. 293 cells were chosen because they are highly transfectable and also permissive to XMRV infection [41] . The full length XMRV Env was unable to induce syncytia formation, presumably due to the presence of an R peptide in the CT (Fig. 4A ). CT624 and CT613, in which the CT of XMRV Env was truncated at the putative R peptide cleavage site and further towards the N-terminus (Fig. 3A) , respectively, elicited apparent syncytia (typically ,30 syncytia per mg DNA, with .6 nuclei per syncytium) (Fig. 4A) . Interestingly, CT609, which contains the first arginine residue of the CT only, showed a much reduced fusion activity as compared to CT624 and CT613 (,5-10 syncytia per mg DNA, with smaller size) (Fig. 4A ). These results were somewhat different from what had been reported for MoMLV, where an identical mutant largely retained the fusogenicity of R peptide-minus mutant [11, 44] .