5. One successful form of gene therapy has involved delivery of an allele for the enzyme adenosine deaminase (ADA) to bonemarrow cells of a child with SCID, and delivery of these engineered cells back to the bone marrow of the affected child.What is one major reason for the success of this procedure as opposed to many other efforts at gene therapy?
(A) The ADA introduced allele causes all other ADA-negative cells to die.
(B) The engineered cells, when reintroduced into the patient, find their way back to the bone marrow.
(C) No vector is required to introduce the allele into ADA-negative cells.
(D) The immune system fails to recognize cells with the variant gene.
(E) The engineered bone marrow cells from this patient can be used for any other SCID patient.