This concept is further supported by studies from both our and other groups that demonstrate increased levels of proinflammatory cytokines, such as tumor necrosis factor alpha (TNF), interleukin-6 (IL-6) or interleukin-1 (IL-1) in the serum of PD patients compared to healthy donors [9,10,11,12,13,14]. Here, TNF is linked to Parkinson disease.