The patient died on 17th September, 1999, four days after the genetically altered virus was injected into his liver. The death was attributed to massive immuno-response caused by adenovirus and resulted in a major blow to the field of gene therapy in USA and elsewhere. All clinical trials at University of Pennsylvanias Institute of Human Gene Therapy were stopped and an investigation was ordered. Several other deaths of patients undergoing gene therapy were not reported and were sometimes attributed to causes other than gene therapy. FDA (Food and Drug Administration) of USA was therefore scrutinizing all gene therapy programmes across USA. As a consequence of the above, the use of adenovirus that was once considered to be the most efficient vector (capable of infecting both dividing and non-dividing cells and capable of expressing the gene within 24 hours, as against 3-6 weeks needed with other vectors), has been curtailed in several laboratories.