Inhibition of MLV/HIV-1 mediated gene transfer by HIV-1 entry inhibitors. Indicated amounts of either (A) AMD-3100 or (B) T20 were added to vector supernatants during the transduction of NIH3T3 cells expressing CD4 and CXCR4. Transfer of the gfp gene was measured and titers are given as percentage of those obtained from untreated supernatants. Inhibition of MLV/HIV-1 vectors was done 2–3 times and had the same outcome. Inhibition of transduction of vectors derived from FLY-HIV-87-GFP cells by AMD3100 was done only once.