ERVW-1 and infection: 32,33 Self-inactivating foamy virus vectors (FVV) based on the prototype foamy virus (PFV) are well characterized34,35,36,37,38 and effective in large animal models of disease.39,40 An historical disadvantage of FVVs is that they induced a marked cytopathic effect (CPE) at a high multiplicity of infection (MOI) in vitro due to the fusogenic nature of the PFV envelope (Env).32 The simian macaque foamy virus envelope (SFVmac Env) has recently been shown to be less fusogenic than that of PFV,41 but has not been described in gene transfer.