The genetic defect - Adenosine Deaminase (ADA) deficiency may be cured permanently by
introducing bone marrow cells producing ADA into cells at early embryonic stages
ADA deficiency was the first to be treated with gene therapy. Lymphocytes from patients blood are grown in culture outside, treated and then returned to the patient. But patient requires periodic infusion as their cells are mortal.
However, if the gene isolated from bone marrow cells providing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.