Despite evidence that a gene-therapy experiment caused a 3-year old boy to develop a leukaemia-like illness, a government advisory panel encouraged the United States Food and Drug Administration (FDA) to reinstate three similar experiments that have been suspended since September, when the child became sick.

The trial involved patients with severe combined immune deficiency (SCID) of the common γ-chain — a rare X-linked disorder in which mature T cells and natural-killer cells fail to develop. Patients received autologous haematopoietic stem cells that had been transduced ex vivo with a retroviral vector encoding the common γ-chain. A report published in April 2002 showed that the deficiency had been corrected in 9 out of 11 patients.

Retroviruses are risky gene-therapy vectors because they can insert into the genome in or near an oncogene or tumour-suppressor gene and disrupt its expression or function. In this case, the retroviral vector inserted into the DNA region that regulates the gene LMO2 — a site associated with a number of T-cell acute-lymphoblastic-leukaemia-specific translocations. The defective immune cells in this patient express LMO2, indicating that the retroviral vector activated the gene.

The retrovirus was probably not the sole factor in the boy's illness, as the family has a history of cancer. The National Institutes of Health's Recombinant DNA Advisory Committee is preparing a broad review of the case this December.