ORF78(ac78)of Autographacalifornica multiple nucleopolyhedrovirus(AcMNPV)is a baculovirus core gene of unknown function.To determine the role of ac78 in the baculovirus life cycle,an ac78-deletedmutant AcMNPV,Ac78KO,was constructed.Quantitative PCR analysis revealed that ac78 is a late gene in the viral life cycle.After transfection intoSpodopterafrugiperdacells,Ac78KO produced a single-cell infection phenotype,indicating that no infectious budded viruses(BVs)were produced.The defect in BV production was also confirmed by both viral titration and Western blotting.However,viral DNA replication was unaffected,and occlusion bodies were formed.An analysis of BVs and occlusion-derived viruses(ODVs)revealed that Ac78 is associated with both forms of the virions and is an envelope structural protein.Electron microscopy revealed that Ac78 also plays an important role in the embedding of ODV into the occlusion body.The results of this study demonstrate that Ac78 is a late virion-associated protein and is essential for the viral life cycle.