One quarter of all hiv carriers now harbor strains resistant to the most potent drug combinations on the market. But a new candidate called T-20 gives those drugs a lift by arresting the deadly virus before it can start infecting cells. Existing aids medications target the enzymes HIV uses to reproduce itself. T-20 works differently. It mimics a crucial, coiled protein molecule that hiv unhooks in order to get into a cell and infect it. T-20 blocks this entry point. "The host cell and the virus can't come close enough together to fuse," says biochemist Sam Hopkins of Trimeris Inc. in Durham, North Carolina, which is developing the drug. In 16-week clinical trials conducted on 55 patients--all of whom had exhausted other treatments--almost two thirds saw significant drops in the concentration of hiv in the blood. In many cases, the virus became undetectable. Researchers anticipate the drug will be generally available within two years and will work on viral strains impervious to existing aids medications. The main drawback is that T-20 is a protein, so it must be injected twice a day; if it were taken as a pill, it would be digested in the gut.