Indeed, the use of neurotrophic factors such as nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) for treatment of neurodegenerative diseases is widely advocated, based mainly on in vitro observations and preliminary success in animal models of AD, PD, Huntington disease, Rett syndrome, traumatic brain injury, and aging [1]–[6]. This evidence concerns the gene BDNF and Rett syndrome.