Truncation of XMRV Env from the C-terminal cytoplasmic tail (CT) promotes SU shedding and syncytia-forming activity While identical in the N-terminal and central regions, including the conserved R peptide cleavage site between 624 and 625, the Cterminal CT of XMRV Env differs from that of MoMLV Env, with a relatively longer length (Fig. 3A ). Here we sought to determine the membrane fusion property of XMRV Env, particularly the effect of CT truncation on cell fusion. We first created a series of truncation mutants in the CT and examined the Env processing and expression by metabolic labeling. 293T cells were pulse-labeled with [ 35-S] Met-Cys for 1 h and chased for 4 h; the XMRV Env proteins in the cell lysates and their SU shed into the culture media were immunoprecipitated with anti-FLAG beads (FLAG is tagged at the N-terminus of SU). As shown in Figure 3B , all the Env constructs were properly processed and expressed in the transfected cells, except CT635 which consistently showed a decreased level of expression of the processed SU (,30% of wildtype) (note the SU subunits of CT624, CT613, CT609, CT608 and CT606 co-migrated with their full length precursors because of their reduced size of precursor, Fig. 3B , upper panel). Of note, CT624, CT613 and CT609 exhibited enhanced SU shedding into culture media as compared to that of wildtype and other mutants (Fig. 3B , lower panel). We also examined the SU surface expression of these Env constructs in 293T cells by flow cytometry using an anti-FLAG antibody, and observed that CT624 exhibited a wildtype level of expression whereas all the other truncation mutants had reduced SU on the cell surface (,50%) (Fig. 3C) . Altogether, these results demonstrate that truncation of the CT of XMRV Env affects SU shedding and surface expression.