Overall this evidence suggests a central role for irisin, In this regard, considering that the hippocampus is one of the principal regions affected by Alzheimer's disease and that exercise causes neurogenesis in humans reducing risk of Alzheimer's [45], Parkinson's, and some other neurodegenerative diseases [26, 46], irisin might be the link between exercise and healthy brain. The gene discussed is FNDC5; the disease is Parkinson disease.