The ability to successfully replace lost nigral dopaminergic neurons in Parkinson's disease (PD) has been clearly shown with fetal ventral mesencephalic transplants, albeit inconsistently. The need to trial this approach with stem-cell-derived neurons is approaching, but it should only commence when all the key issues have been adequately resolved.
Barker, R. A. (2014). Developing stem cell therapies for Parkinson’s disease: Waiting until the time is right. Cell Stem Cell. Cell Press. https://doi.org/10.1016/j.stem.2014.09.016