PubMed lists 73,898 citations under the search term, ‘Parkinson’s disease treatment,’ but only 401 citations, or 0.5%, under the search term, ‘Parkinson’s disease treatment and blood–brain barrier drug delivery.’ GDNF has been available for treatment of PD for 30 years, but has never been approved for PD owing to inadequate brain delivery. This evidence concerns the gene GDNF and Parkinson disease.