Closing the loop on a 10-year quest, Paul Maddon has seen his company participate in the completion of a hunt he started in the mid-1980s. Maddon, chairman and CEO of Progenics Pharmaceuticals Inc., cloned the CD4 receptor in 1985. He also showed that CD4 was the receptor for HIV, and that it was necessary - but not sufficient - to allow HIV to enter cells. That set off a search for the other cofactor for HIV infection.

Reporting separately in Nature and in Science, Progenics researchers and collaborators, and NIAID scientists have now documented the existence of the cofactor. The investigators showed that entry of HIV into macrophages and T