Polynucleotide immunization has been employed as a means of inducing immune responses through the introduction of antigen-encoding DNA. While immunization against specific tumor antigens may be achieved through this strategy, various candidate tumor antigens may not be approached via DNA-based vaccines as they represent transforming oncogenes. As an alternative approach, we have explored the utility of mRNA vectors for polynucleotide immunization. The transient expression achieved by mRNA may provide an efficient and safe system for stimulating immune responses to tumor-specific antigens. Our previous work demonstrated that a self-replicating RNA enhances the magnitude and duration of transgene expression for this application. Here we further modify the vector for optimal use in gene therapy through the incorporation of untranslated regions flanking the encoded transgene. The β-globin 5′ and 3′ untranslated regions (UTRs) were inserted directly flanking the luciferase gene in both nonreplicative and replicative RNA constructs. In both cases, elevated and prolonged levels of luciferase expression were detected from the β-globin UTR-flanked luciferase as compared to luciferase without these sequences. These modifications improve the ability of replicative RNA vectors to produce high, yet transient transgene expression for cancer immunotherapy strategies.