Recent studies have consistently reported altered levels of BDNF in the circulation (i.e., serum or plasma) of patients with major depression, bipolar disorder, Alzheimer's disease, Huntington's disease, and Parkinson's disease (52), suggesting that BDNF levels in serum and plasma can be used to reflect the level of BDNF in peripheral blood. This evidence concerns the gene BDNF and Parkinson disease.