In an aging research building at the University of Southern California, a $14.5 million biomedical experiment is under way that until a few years ago would have made many AIDS researchers snicker at its ambition. Mice are the main research subjects (for now), and some 300 of them live in a room the size of a large walk-in closet. Signs plastered to the room's outer door include blaze-orange international biohazard symbols and a blunter warning that says, "This Room Contains: HIV-1 Infected Animals." Yet the hazard is accompanied by an astonishing hope. In some of the infected mice, the virus appears to have declined to such low levels that the animals need no further treatment.

This is a feat that medications have not accomplished in a single human, although daily doses of powerful anti-HIV drugs known as antiretrovirals can now control the virus and stave off AIDS for decades. Every person who stops taking the drugs sees levels of HIV skyrocket within weeks, and immune destruction follows inexorably. The lack of a cure--a way to eliminate HIV from an infected person or render it harmless--remains an intractable and perplexing problem.