Entry of HSV-1 into cells. The initial contact of virus with cell is usually the binding of virus to heparan sulfate chains on cell surface proteoglycans (HSPG). Either of the viral glycoproteins gB or gC can mediate this binding. Then viral gD can bind to any one of several entry receptors, including HVEM (I), a member of the TNF-receptor family; nectin-1 or nectin-2 (II), two related members of the immunoglobulin superfamily; or sites generated in heparan sulfate by the action of specific 3-O-sulfotransferases (III). The binding of gD to one of its receptors activates viral membrane-fusing activity, which requires the action of gB and gH-gL as well as gD and a gD receptor. Fusion between the viral envelope and cell membrane liberates the viral nucleocapsid and tegument into the cell cytoplasm.