The plasmids pCMV-BLVenv-RRE and pRSV-Rev have been described [17, 47] . To construct pCMV-HTLVenv-RRE the BLV env sequences in pCMV-BLVenv-RRE were replaced by a HTLV-1 env coding region amplified by PCR from pHTE-1 [48] . To construct the plasmid pMAL-gp30hairpin (MBP-BLV-hairpin), a modified fragment of the MBP open reading frame from pMAL-c2 (New England Biolabs) with a 59 BglII site and a 39 PvuII site replacing a SacI site was PCR amplified. A fragment of BLV env encoding amino acid residues 326 to 418 was PCR amplified with a 59 PvuII site and a 39 PstI site. The PCR fragments were ligated into a pMAL-c2 backbone digested with BglII and PstI. The Quikchange mutagenesis kit (Stratagene) was used to introduce point mutations into pCMV-BLVenv-RRE and pMAL-gp30hairpin following the manufacturer's instructions.