In gene therapy of Adeosine Deaminase (ADA) deficiency, the patient requires periodic infusion of genetically engineered lymphocytes because:
Gene isolated from marrow cells producing ADA is introduced into cells at embryonic stages.
Lymphocytes from patient's blood are grown in culture outside the body
Genetically engineered lymphocytes are not immortal cells.
Retroviral vector is introduced into these lymphocytes.
In ADA deficiency, lymphocytes are given but these cells are immortal and hence periodic infusion is needed. Option 2 is correct.