Figure 2. In vivo versus ex vivo gene therapy.

(A) In vivo gene therapy is based on direct gene transfer into the cells requiring the replacement therapy. For CNS diseases, this implies direct injection of vectors into the CNS. (B) For ex vivo gene therapy, the gene transfer is performed in a cell type outside the body (e.g., blood cells), and the genetically modified cells are injected into the deficient recipient. The genetically modified cells can then circulate within the bloodstream, migrate through tissues, and deliver their therapeutic load to neighboring cells — a process known as cross-correction.