BDNF levels have been hypothesized to be reduced in Alzheimer's disease, Parkinson's disease, and Huntington's disease.21–23 BDNF depletion has also been suggested to play a role in glaucoma,24 with evidence for impaired retrograde BDNF transport from the brain to the retina demonstrated in experimental models of ocular dysfunction.25–27. This evidence concerns the gene BDNF and Parkinson disease.