The transduced cells are grown in culture, collected  in large amounts, and then introduced into the      patient at various intervals; with the hope that the  cells will be maintained and the disease that is being treated will be corrected.   

Patients with genetic diseases that respond to bone  marrow transplantation are treated by ex vivo gene therapy.   

Bone marrow transplantation acts as a therapeutic    procedure for different genetic diseases, bone marrow  contains totipotent embryonic stem cells at a    frequency of 10-4 to 10-5. These embryonic cells can     divide and differentiate into various important cell    types, including B and T lymphocytes, macrophages, Red blood cells, platelets and bone cells.   

Ex vivo gene therapy is a procedure in which genetically engineered totipotent stem cells are transplanted into a patient, where they would provide the missing cell type or gene product.