The virus was still present in the cells she donated, and killed her brother. But just like the insulin injections of a diabetic, they must be repeated at frequent intervals. It must be inserted in cells that can take up longterm residence in the patient. So far, this means removing the patient's own cells , treating them in tissue culture, and then returning them to the patient. All these requirements seem to have been met for SCID therapy using a retrovirus as the gene vector .
Retroviruses have several advantages for introducing genes into human cells.
Their envelope protein enables the virus to infect human cells. RNA copies of the human ADA gene can be incorporated into the retroviral genome using a packaging cell. Once the virus has infected the target cells, this RNA is reverse transcribed into DNA and inserted into the chromosomal DNA of the host. But someday there may be a way to make genetically-engineered T cells live indefinitely by getting them to express telomerase . See the discussion .