HeLa cells were maintained in Dulbecco's modified Eagle medium supplemented with 10% foetal bovine serum (FBS). For syncytium formation assays, HeLa cells were transfected with equal quantities of pCMV-BLVenv-RRE (or empty pcDNA 3.1 for mock samples) and pRSV-Rev using the GeneJuice transfection reagent (Novagen). After 24 h, 3610 5 transfected cells were added to 7.0610 5 non-transfected HeLa target cells in 6-well dishes (Nunc), and cocultured for 16 hrs, in the presence of peptides where appropriate. The cells were washed with phosphatebuffered saline pH 7.4 (PBS), fixed using 3% paraformaldehyde in PBS, and stained with Giemsa. Assays were performed in triplicate. To calculate fusogenic indices, the fraction of nuclei contained within syncytia in a 20x light microscope field was expressed as a percentage of the total number of nuclei within the field.