The baculovirus expression vector system is a helper-independent system that has found extensive use in the past decade for the expression of heterologous genes. Its popularity stems from a combination of high levels of expression with the ability to carry out most eukaryotic posttranslational modifications in an authentic manner. As a consequence, the system has an excellent track record for the production of functional eukaryotic gene products. (See ref. 1 for a detailed recent review of the expression vector system and the basic biology that underlies it.) Baculoviruses are a large family of DNA viruses that have only been isolated from arthropods, with the majority found in insects. Autographa californica nuclear polyhedrosis virus (AcMNPV), the prototype baculovirus strain and the virus most commonly used in expression vector systems, infects a number of lepidopteran species. Like most baculoviruses, it has two distinct morphological types. The budded virus form comprises single virus particles surrounded by a lipid envelope, whereas the occluded virus form comprises multiple virus particles embedded in a matrix composed almost exclusively of a protein called polyhedrin. The bodies containing the occluded virus particles are known as occlusion bodies or polyhedral inclusion bodies. Up to 100 such occlusion bodies may form in the nucleus of an infected cell.