The most potent neurotrophic factor for PD is GDNF (Ibáñez and Andressoo, 2017), and clinical trials of trans-cranial delivery of GDNF via ICV injection (Nutt et al., 2003) or convection enhanced diffusion (CED; Lang et al., 2006) have been attempted. The gene discussed is GDNF; the disease is Parkinson disease.