A chink in HIV's armour?

Sir-Elegant work has established that the human immunodeficiency virus (HIV) infects susceptible cells following interactions between the viral envelope glycoprotein (gpl20) and a cell-surface receptor (CD4) (refs 1,2). During the development at Celltech of a process for the production of recombinant HIV-1 gpl20 from Chinese hamster ovary cells for the MRC Aids Directed Programme, we have identified a specific proteolytic cleavage event of the gpl20 molecule under certain cell culture conditions. Cleavage becomes apparent only after SDS-PAGE of the purified protein under reducing conditions, indicating that the two cleavage products, relative molecular mass 70,000 (70K) and 50K, are held together by a reducible disulphide bridge.