A modified virus including one or more non-native polypeptides, and the use of such viruses in therapy, particularly in the treatment of tumours or other cancerous cells are disclosed. The polypeptides includes one or more framework moieties each containing one or more binding moieties, and is capable of being expressed in the cytoplasm and nucleus of a mammalian host cell in a conformation which is maintained in the absence of a ligand for the binding moieties. The conformation allows the binding moieties subsequently to bind with the ligand, and the polypeptide is capable of transport through the nuclear membrane, wherein the modified virus has an altered tropism conferred by the binding moieties.