Kathy High doesn't sleep well these days. A chronic insomniac, the University of Pennsylvania hematologist now gets even less rest than usual. The reason? Stress. High does gene therapy.

These are not easy times for High and other researchers trying to cure patients with DNA. A few hundred yards down the road from High's Children's Hospital of Philadelphia office, over in Penn's main hospital, 18-year-old Jesse Gelsinger died last September after receiving gene therapy for a rare liver disorder. The teenager's death prompted highly public soul-searching by the gene-therapy community and intense scrutiny by the Food and Drug Administration. In January, the FDA put all of the human trials run out of Penn's Institute for Human Gene Therapy, including the one Gelsinger had volunteered for, on indefinite hold.