5-Helix, an artificial protein designed by Peter S. Kim and his colleagues at the Whitehead Institute at MIT (Cambridge, MA), prevents HIV infection by blocking entry of the virus into human cells (Published online on January 11, 2001 in Science 10.1126/science.1057453). By binding to a highly conserved helical region of the HIV coat protein gp41, 5-Helix prevents the virus from fusing with the membrane of its host cell. The protein works in nanomolar concentrations in vitro, and shows “broad spectrum antiviral activity against a wide range of HIV isolates,” says Kim. Existing drugs to combat HIV, including reverse transcriptase and protease inhibitors, block HIV replication only after the virus has already infected a cell. By preventing viral entry, 5-Helix provides an alternative antiviral mechanism that could prove effective as a salvage therapy when available drugs fail because of intolerable side effects or the emergence of drug-resistant HIV strains. “5-Helix is a very stable protein and is not that far removed from testing in animals,” says Kim.