Some viruses utilize a single molecule as a receptor for entry into the host cell, while many viruses require co-receptor(s) localized near the receptor for complete entry. Identification of viral entry receptors that are composed of membrane proteins, lipids, or carbohydrates is important for examining the life cycle of a virus and for further developing entry inhibitors. However, receptors or co-receptors of several viruses have been difficult to identify because of the lack of reliable cell culture systems, an insufficient amount of native viral particles, or difficulty in handling because of the requirement for biosafety level (BSL)-3 or -4 containment. Therefore, several surrogate systems have been developed to study the initial step of infection. One of the most primitive assays is a binding assay. Purified soluble envelope proteins, viral-like particles which are produced in insect cells by baculoviral vectors, and authentic viral particles obtained from patients have been used to study the mechanisms of viral attachment and to identify binding receptor molecules. However, these binding assays cannot be used to analyze further steps of infection such as fusion and penetration. A cell fusion assay was established to examine the membrane fusion activity of viral envelope proteins. This assay is sensitive and can easily determine cell fusion using reporter genes. Pseudotype virus systems based on vesicular stomatitis virus (VSV), influenza virus, retroviruses, and lentiviruses have also been established to examine entry mechanisms and to identify putative entry receptors for targeted viruses ( Table 1) . Pseudotype viruses have also been applied in neutralization tests for antibodies and vaccine development ( Table 1) . As for the application of a pseudotype virus system for VSV, a recombinant virus system with a heterologous viral envelope gene together with a reporter gene encoded into its own genome instead of the G gene has also been developed.