To examine the possibility that the lack or reduced fusion for CT609, CT608 and CT606 might be due to a block at hemifusion, we treated co-cultured target and effector cells with chlorpromazine (CPZ, 0.2-0.5 mM), a membrane permeable reagent that promotes the transition from hemifusion to full fusion [46] , but observed no apparent increase in fusion for any of these Env constructs (data not shown). These results suggest that the fusion suppression in these XMRV Env constructs unlikely takes place at the hemifusion step. Overall we conclude that, distinct from JSRV Env, severe truncation of the CT of XMRV Env towards the MSD does not further enhance but rather impairs the Env fusogenicity. The reason for the decreased fusogenicity of CT609, CT608 and CT606 remains unclear, but is likely related to reduced surface expression or/and altered Env conformation (see Discussion).