Some flavonoids such as luteolin, apigenin, and myricetin increase the tyrosine hydroxylase activity and the dopamine level in the brain, which is very important in the case of Parkinson’s disease, while a decrease in the BACE-1 activity by chrysin, catechins, quercetin, eriodyctiol, and biochanin A is crucial for reducing amyloid-β aggregation during the development of Alzheimer’s disease. The gene discussed is TH; the disease is early-onset autosomal dominant Alzheimer disease.