HIV, the etiologic agent of AIDS, is a retrovirus of the family Lentiviridae , first isolated in 1983 by the group of Luc Montagnier at the Pasteur Institute of Paris (1 ). In the following years, much effort has been, and still is, focused on the search for antiviral drugs that would help to control the course of the disease in infected individuals. To assess the efficacy of those drugs, in vivo clinical markers of virus replication needed to be defined. These markers were for some years, surrogate, e.g., CD4 cell numbers, one of the main target cell types in the HIV replication cycle. More cumbersome methods were also used, such as the in vitro culture of plasma virus on donor peripheral blood lymphocytes (PBMC), with variable results. None of these techniques, however, could provide an accurate measure of virus replication. A major breakthrough in this field was the advent of methods that would allow for a direct quantification of circulating HIV. Viral load determinations soon proved to be an invaluable tool both in the clinical management of HIV-infected individuals and in the monitoring of therapeutic efficacy for commercially available or experimental antiviral drugs (2 ).