We next asked if HIV-1 infection could modulate the expression of the cathepsin B inhibitor cystatin C, since the latter is the major extracellular inhibitor of cathepsin B. Expression of cystatin C and the ratio between secreted cathepsin B and cystatin C determines the amount of potentially active cathepsin B in the extracellular medium. This evidence concerns the gene CST3 and HIV-1 infection.