The present invention provides mutant viruses with a decreased ability toblock nuclear transport of mRNA or protein in an infected cell which areattenuated in vivo. The mutant viruses of the present invention may also becapable of triggering the anti-viral systems of normal host cells whileremaining sensitive to the effects of these systems. The present inventionfurther provides for the use of the mutant viruses in a range of applicationsincluding, but not limited to, as therapeutics for the treatment of cancer andinfections, as vaccines and adjuvants, as viral vectors, and as oncolytic andcytolytic agents for the selective lysis of malignant or infected cells.