Publisher Summary The expression of heterologous genes in insect cells using baculovirus vectors is commonly used to achieve a high level of recombinant protein production. The expression system exploits the promoters from a number of hyperexpressed virus genes that are activated in the very late stages of the Autographa californica nucleopolyhedrovirus (AcMNPV) replication cycle. The most widely used of these are the polyhedrin gene (polh) and p10 (p10) gene promoters. Replacement of either the polh or p10 coding region with that of a foreign gene results in the formation of a recombinant virus with the foreign gene under control of the hyperexpressed promoter. Recombinant viruses may be propagated in insect cells cultured in vitro, for example, Spodoptera frugiperda or Trichoplusia ni cells, or may be used to infect insect larvae. In the latter case, recombinant viruses are usually prepared in which the polh and p10 remain intact, so that larvae may be infected by the natural route of feeding upon a polyhedra-containing diet. In this case, the foreign gene is expressed under control of a copy of the polh or p10 promoter, located in a nonessential region of the virus genome, for example, just upstream of the polh promoter.