Noticeably, the increased fusion activity of CT624 and CT613, and to lesser extent of CT609, correlated with the enhanced SU shedding of these mutants in culture media (Fig. 3B) . These results are similar to our previous findings made on JSRV Env, severe truncation of which led to pronounced SU shedding accompanied with greatly increased fusogenicity [19] . Interestingly, we observed that the tailless CT608 and CT606 mutants were virtually fusiondefective, possibly due to their truncation into the MSD and/or reduced surface expression ( Fig. 3A and 3C ). We further treated the individual Env-expressing cells with a low pH buffer (pH 5.0) for 1 min or 5 min (pictures not shown), but did not observe apparent effect on syncytia induction of any of these constructs, supporting the above conclusion that XMRV Env-mediated fusion and cellular entry is pH-independent.