What is gene therapy, and what was the first disease treated by it?

Gene therapy is the correction of a genetic defect by introducing a functional copy of the gene into the patient's cells. The first attempt, in 1990, treated adenosine deaminase deficiency, a form of severe combined immunodeficiency, by returning the patient's own lymphocytes carrying a functional ADA gene.

No citable source has been recorded for this record. Treat it as practice material, not as fact.

Explanation

Because the lymphocytes are not immortal the treatment has to be repeated; introducing the gene at an early embryonic stage would be a permanent cure.