Pseudoviruses or pseudotypes possess the core and genome structure of one virus and the envelope glycoproteins of another. The term was first used for envelope-defective strains of Rous sarcoma virus bearing leukemia virus envelopes, pioneered by Peter Vogt and Hidesaburo Hanafusa. Pseudoviruses later led to the development of retroviral vectors for gene transfer. Retroviruses and lentiviruses expressing cloned envelope genes have proved useful for determining cell surface receptors for viruses, and also for measuring the titer and breadth of neutralizing antibodies. Vesiculo-stomatitis virus pseudotypes were pioneered by Jan Zavada and modern forms look promising as replication-competent yet apparently safe vaccines, eg. against Ebola. The assembly of pseudoviruses bearing the envelopes of highly pathogenic viruses provides a safe and simple means for titrating the neutralizing properties of sera and monoclonal antibodies derived from patients naturally infected with enveloped viruses such as HIV, H5N1 Influenza, Rabies and Ebola, as well as antibodies elicited by candidate vaccines.