After a dozen years of human gene therapy trials characterized by minimal gene correction and disappointing clinical impact, the field of gene therapy received some good news in 2000. Infants with X-linked severe combined immunodeficiency who received retroviral gene addition to cells from their bone marrow developed impressive immune reconstitution. During the following 2 years, additional patients were treated and the news was even better-babies receiving gene therapy had sustained T-cell production and in several cases developed better cell function than most patients treated with standard bone marrow transplants. Unfortunately, bad news followed. Three of the patients experienced leukemic T-cell expansions, found to be associated with retroviral insertions into genomic DNA. Where does the field stand today?