Plasmid pFUGW was constructed by inserting the following into the multicloning site of HR'CS-G: HIV-1 flap sequence PCR-amplified from the HIV NLA4.3 genome, the human polyubiquitin promoter-C (gift of L. Thiel, Amgen), the EGFP gene, and the WRE (woodchuck hepatitis virus posttranscriptional regulatory element) (gift of D. Trono, University of Geneva). Lentiviruses can be produced by cotransfecting the HIV-1 packaging vector Delta8.9 and the VSVG envelope glycoprotein into 293 fibroblasts.