In 2003, the field of gene therapy faced serious adverse events, apparently arising in part from insertional activation of the LMO2 gene in a clinical gene therapy trial in Paris that sought to correct deficiency of the interleukin-2 common -chain receptor (IL2RG). Although this remains the only human gene therapy trial reporting such side effects (including an almost identical trial in London), and the molecular mechanisms involved in the leukemogenesis are still being hotly debated, the development of leukemia in three participants and the previous demonstration by Baum's group of a mouse leukemia associated with replication-defective Moloney leukemia virus vectors1 added momentum to the development of lentiviruses as vectors for hematopoietic stem cell (HSC) transduction.