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Glycoprotein B (gB) of Herpes simplex virus type 1 (HSV-1) plays an essential role in viral entry. A set of more than 100 HpaI (GTTAAC) linker insertion mutations and their derivatives were isolated in plasmids specifying the gB coding and flanking sequences. Mutations including addition, deletion and nonsense mutations at 34 independent sites were identified by DNA sequence analysis of 48 plasmids. A map was constructed for the ability of addition mutants to complement a gB-null virus. The expression of gB activity for some plasmids was temperature-dependent. Many complementation-negative plasmids inhibited the complementation activity of a plasmid specifying wild-type gB, suggesting an interaction between active and inactive molecules to form oligomers. The interaction was localized to 328 of the total of 904 amino acids comprising gB. Partial Endo H digestion of nonsense polypeptides revealed that five of the six potential N-linked oligosaccharide sites are glycosylated; the most C-terminal site appears not to be glycosylated. A number of mutations, including some on the cytoplasmic side, were identified that blocked processing, transport and secretion. Addition mutations that blocked processing of membrane polypeptides also blocked processing and secretion when combined into a nonsense mutant that by itself was processed and secreted. The previously predicted membrane spanning domain and the membrane orientation of the N-terminal portion of gB were confirmed.