The invention concerns the use of a nucleic acid/cationic polymer complex, preferably polyethyleneimine (PEI for preparing a composition for intraventricular stereotactic screening of stem cells of the brain for preparing a medicine for treating neurodegenerative and/or demyelinating disease. The invention further concerns a method for obtaining an animal whereof the genome of stem cells of the brain are modified by using said complex. The invention also concerns a method for obtaining an animal for screening compounds designed to modify the disposition of stem cells of the brain.