If the virus could not reproduce normally in a given well, it suggested that the missing protein was one of those it needed.

Of the 273 human proteins identified, only 36 had been previously found by other methods.

The virus, which is itself only a short string of genetic material inside a protective capsule, can make only 15 proteins, so it has to adopt human proteins to its own use.

The advantage of targeting human proteins is that the virus would presumably not be able to mutate to avoid drugs that block them, Dr. Elledge said. Right now, virus strains evolve resistance to antiretroviral drugs, which attack the 15 proteins made by the virus itself, like reverse transcriptase and protease. The mutations force AIDS patients to switch drug regimens  not always successfully.

The disadvantage is that blocking human proteins can, obviously, be fatal to humans. But, as Dr. Gallo pointed out, cancer therapy works that way  doctors try to block proteins that feed fast-growing tumor cells without killing too many other fast-growing cells, like those in the bone marrow.

Right now, Dr. Elledge said, only one drug that targets one of the known human proteins, a receptor called CCR5, has been developed, and it has just won approval.

The new screening technology, known as siRNA, is now used in many laboratories, so this work could theoretically have been done elsewhere, or by using older, more laborious methods.

Dr. Elledge said he benefited from working at Harvard, which could afford the expensive robotics and imaging technology needed.