In 1990 Ashanti DeSilva, a four-year-old with severe combined immune deficiency (SelD), had her white blood cells removed and replaced with a normal copy of the defective gene. SClD is a life-threatening disease in which patients lack a healthy immune system due to the inability to produce an important enzyme (adenosine deaminase). Although the treatment proved safe and her immune system was strengthened, the treated cells failed to give rise to additional healthy cells. In order to maintain normal levels of adenosine deaminase, DeSilva must take doses of the enzyme itself.
