The use of targeting knottin polypeptides provides a means of selectively infecting target cells presenting species bound by the knottins. The targeted viruses may be used as cytotoxic agents, for example, as oncolytic viral therapeutics. Alternatively, the knottin-targeted viruses may be used to transform target cells in a gene therapy or immunotherapy context. An effective retargeted measles virus directed to integrins is demonstrated.
