GDNF and Parkinson disease: These controversial results suggest that there are other tactical variables not inherent to the disease or resistance to GDNF that should also be taken into account in the treatment of PD patients; for instance, (i) optimal timing of drug administration during the natural course of the disease; (ii) route of administration; (iii) anatomical site of administration; (iv) administration period; (v) type of associated therapies in addition to GDNF treatment, and (vi) delivery system efficiency.