At present, the most widely used baculovirus expression system uses a lytic virus known as Autographa californica nuclear polyhedrosis virus (AcMNPV; hereafter called baculovirus). This virus is the prototype of the family Baculoviridae. It is a large, enveloped, double-stranded DNA virus that can infect arthropods. AcMNPV gene expression is temporally regulated in an orderly cascade proceeding from early to late phases, followed by very late phase. Viral DNA replication is initiated during the late and very late phases, allowing the production of its progeny viruses.