Although this evidence suggests that neurotrophic factor addition may be a potential method for improving the survival and health of the transplanted cells, clinical trials performed with such biomolecules delivered into the brain tissue either as a protein or gene therapy [90,91] gave inconclusive results [92], thus mitigating the enthusiasm about the possible role of GDNF in the PD treatment. The gene discussed is GDNF; the disease is Parkinson disease.