XMRV Env was initially engineered to contain a FLAG tag at both N-and C-termini by using pcDNA3.1-VP62 (gift of Robert Silverman) [37] as a template for PCR, and cloned in a pCIneo expression vector (Promega, Madison, WI), the resulting construct was referred to as pCIneo-F-Xenv-F. To create the N-terminal FLAG-tagged XMRV Env wildtype and CT truncations, the pCIneo-F-Xenv-F construct was used as a template, with the following lower primers being used for PCR amplification (Not I sites are underlined): XMRV Env, 59-ATCGGCGGCCGCT-CATTCACGTGATTCCACTTC-39; CT635, 59-TTCTGCG-GCCGCTCATGATTTGAGTTGGTGATA-39; CT624, 59-C-TGTGCGGCCGCTCACAGGGCCTGCACTACCGA-39; CT-613, 59-AATTGCGGCCGCTCAAAACTGGACCAAGCGGT-TG-39; CT609, 59-TACAGCGGCCGCTCAGCGGTTGAGA-ATACAGGGTCCGA-39; CT608, 59-AAACGCGGCCGCT-CAGTTGAGAATACAGGGTCCGA-39; CT606, 59-GACC-GCGGCCGCTCAAATACAGGGTCCGAAGA-39. The pCI-neo-10A1, pMD.G and pCIneo-Ebola GP expression vectors have been previously described [18] .