The simple structure and rapid high-titer growth of VSV in mammalian and many other cells has made it a useful tool in the fields of cellular and molecular biology and virology, and this was further strengthened with the establishment of the reverse www.frontiersin.org genetics system for VSV. Recombinant VSV in which native envelope G protein is replaced with a foreign reporter gene such as a fluorescent reporter protein, luciferase, or secreted alkaline phosphatase (SEAP) can normally bud from producing cells even in the absence of G protein, and heterologous viral envelope proteins are incorporated into the virion. Previous studies demonstrated that VSV forms a "pseudotype" when a cell is co-infected with VSV and other enveloped viruses (Huang et al., 1974; Witte and Baltimore, 1977) . A pseudotype virus is defined as a viral particle harboring other types of viral envelopes or host cellular proteins with or without its own envelope. By virtue of these characteristics of VSV, pseudotype virus systems, in which VSV G proteins are completely replaced with other types of viral envelope proteins, have been established (Figure 1) . Up to the present, numerous types of pseudotype viruses have been constructed with heterologous viral envelope proteins and used in studies examining the entry of viruses, for identification of novel viral receptors, for development of neutralization tests, and as vaccine vectors ( Table 1 ). In particular, availability of pseudotype viruses has been useful in the study of several high-risk viruses that require high-level containment facilities, i.e., in the handling of BSL-3 or -4 viruses. Infectivity of these pseudotype viruses can be easily and quantitatively evaluated by measurement of the reporter gene activities. A recombinant virus system, which encodes a heterologous viral envelope gene instead of an envelope gene in its own genome, has also been made available by establishment of reverse genetics (Figure 2 ). This recombinant virus is replication-competent both in vitro and in vivo and can contribute to the study of targeted viruses that inefficiently replicate in experimental systems. Although the pseudotype virus is limited to single-step infection and therefore provides a poor model for actual infection processes, the recombinant virus is a far more authentic and powerful tool for investigating targeted viral infection. Currently, this system is applicable to the VSV or other FIGURE 1 | Schematic representation of the production of pseudotype VSV. Producer cells were transfected with an expression plasmid encoding foreign envelope genes and then infected with a VSV G-complemented pseudotype virus (*G-VSVΔG). The pseudotype virus released from the producer cells infected target cells but was not able to produce infectious progeny viruses. several viruses and not to retroviruses or lentiviruses. Recombinant VSV can be produced in various cells without regard to transfection efficiency; on the other hand, recovery of pseudotype VSV as well as pseudotype retroviruses or lentiviruses is restricted to 293T or some other type of cells that exhibit a high competency of transfection. Recombinant VSV could also lead to the induction of cellular and humoral host immunity (Schnell et al., 1996) .