Through fusion to TTC, a number of functional, therapeutic proteins have been delivered to motor neurons in a cell-specific manner in vivo for proof-of-concept treatment of amyloid lateral sclerosis (ALS) and Parkinson’s Disease, such as human insulin-like growth factor-1 (hIGF-1; [15]), human Cu/Zn superoxide dismutase (hSOD-1; [16]) and glial cell line derived neurotrophic factor (GDNF; [17]). This evidence concerns the gene GDNF and Parkinson disease.