Complex sentence: During In vivo gene transfer, the genes are transferred directly into the tissue of the patient and this can be the only possible option in patients with tissues where individual cells cannot be cultured invitro, in sufficient numbers (i.e. brain cells).
Split into simpler sentences: During In vivo gene transfer the genes are transferred directly into the tissue of the patient. This can be the only possible option in patients with tissues where individual cells cannot be cultured outside the body in sufficient numbers (i.e. brain cells).