Which of the following could be a permanent cure for ADA deficiency in a patient?
 

In somatic gene therapy, a functional ADA cDNA (using a retroviral vector) is introduced into the lymphocytes, which are subsequently returned to the patient. As these cells are not immortal, the patient requires periodic infusion of such genetically engineered lymphocytes. However, if the gene isolate from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.