Abstract: The present invention provides mutant viruses with a decreased ability to block nuclear transport of mRNA or protein in an infected cell which are attenuated in vivo. The mutant viruses of the present invention may also be capable of triggering the anti-viral systems of normal host cells while remaining sensitive to the effects of these systems. The present invention further provides for the use of the mutant viruses in a range of applications including, but not limited to, as therapeutics for the treatment of cancer and infections, as vaccines and adjuvants, as viral vectors, and as oncolytic and cytolytic agents for the selective lysis of malignant or infected cells.