Alphavirus-based vectors have received considerable attention in the fields of antigen (Ag) presentation, epitope mapping, vaccinology, and gene therapy (1-12), because of their relative ease of manipulation, purification to high titers, and ability to express large amounts of protein (13). Several members of the Alphavirus genus, including Sindbis virus (SIN), Semliki Forest virus, and Venezuelan equine encephalitis virus, have been modified genetically for use as viral expression vectors for foreign Ags (1, 3, 8, 13-17). SIN virions are comprised of an icosahedral nucleocapsid surrounded by a plasma-membrane- derived envelope containing two viral glycoproteins, E1 and E2 (18). The nucleocapsid contains a single stranded, positive (+) sense RNA gene of approx 11,700 nucleotides, which is capped and polyadenylated, and associated with 240 copies of the viral capsid protein.