The Second Key

For a decade, AIDS researchers have known one thing about how the virus infects human immune cells: it binds to a receptor molecule called cd4 on the cell surface. But they’ve also known that cd4 is not enough. Even if an animal has been engineered to produce human cd4 on its immune cells, those cells cannot be infected with hiv. Last May, biochemist Ed Berger of the National Institutes of Health and his colleagues announced that they had finally succeeded--where many before them had failed--in identifying an elusive coreceptor that makes certain human cells peculiarly vulnerable to hiv. Their finding unleashed a storm of papers that not only help clarify our understanding of aids but also may lead to new treatments for the disease.