The T4 glycoprotein is a cell-surface receptor for the AIDS virus.

Order of authorship in the original paper: McDougal, Maddon, Dalgleish. The authors discovered that the T4 lymphocyte cell has an outer glycoprotein on its surface that specifically acts as the receptor for HIV. Without first attaching to this receptor HIV cannot dock onto and penetrate the T4 lymphocyte.

(Thanks to Juan Weiss for this entry and its interpretation.)