The alphaviruses Semliki Forest virus (SFV) and Sindbis virus (SIN) have been used frequently as expression vectors both in vitro and in vivo. In most cases, these systems consist of replication-deficient vectors that require a helper vector for packaging of recombinant particles. Recently, replication-proficient vectors have also been engineered. Alphaviral vectors can be used in various forms as nucleic-acid-based vectors (DNA and RNA) or infectious particles. High-titer viral production is achieved in less than 2 days and usually requires no further concentration or purification The broad host range of alphaviruses facilitates studies in mammalian and nonmammalian cell lines, primary cells in culture, and in vivo. The strong preference for expression in neuronal cells has made alphaviruses particularly useful in neurobiol. studies. Unfortunately, their strong cytotoxic effect on host cells, relatively short-term transient expression patterns, and the reasonably high cost of viral production remain drawbacks. However, novel mutant alphaviruses have demonstrated reduced cytotoxicity and prolonged expression. In particular, membrane proteins (which are generally difficult to express at high levels in recombinant systems) have generated high yields and facilitate applications in.