Since a poor Th1 response and a dominant Th2 response have been implicated in the pathogenesis and progression of HIV infection (Clerici and Shearer, 1993, 1994), HTLV-2 priming for a Th1 response via up-regulation of IFN-γ expression may contribute to the “protective” effect of HTLV-2 infection on HIV-1 disease progression. This evidence concerns the gene IFNG and HIV infectious disease.