The present invention provides for a mutant reovirus and a genetically-engineeredISVP. Specifically, the present invention provides a mutant reovirus that lacksor exhibits a reduced expression of the sigma3 polypeptide or that lacks a functionalsigma3 polypeptide. The mutant reovirus described therein can be used to generateand stably propagate ISVP. Compositions including the mutant reovirus and methodsof using such mutant reovirus are also provided.