Recent studies show that HIV-1 replication can be dramatically curtailed, if not completely arrested, with potent, rationally-designed drug combinations. The benefits of combination therapy, however, are greatly diminished in patients who have received previous antiretroviral therapy. Although 13 antiretroviral drugs are available, there is considerable cross-resistance within each class of inhibitors. Preliminary data suggest that some of the current drug-resistant HIV-1 isolates are already cross-resistant to many of the drugs in clinical development. This proposal plans to identify patterns of RT and protease mutations developing in HIV-1 isolates from patients receiving treatment with the majority of available RT and protease inhibitors. This application will analyze (I) published HIV-1 RT and protease sequences, (II) RT and protease sequences obtained in ACTG trials, and (III) RT and protease sequences obtained during the clinical management of >500 HIV-1-infected patients in the San Francisco Bay area. Infectious biological and recombinant molecular clones will be created from these isolates; their susceptibility to available and experimental antiretroviral drugs will be assessed; and, the role of specific multidrug-resistance mutations in site-directed mutagenesis and virus passage experiments will be assessed. An understanding of the genetic changes responsible for multidrug-resistance is essential for the development of new antiretroviral drugs, for designing optimal drug combinations, and for the clinical management of individual patients. A set of well-characterized multidrug-resistance HIV-1 isolates will also be of the utmost value to researchers in the areas of antiretroviral drug development and drug resistance.