TNF and Parkinson disease: Serum TNF-α level was found to be elevated in PD patients (Müller et al., 1998; Gruden et al., 2012; Koziorowski et al., 2012; Lindqvist et al., 2012; Williams-Gray et al., 2016; Dobbs et al., 1999) and a meta-analysis in 2016 included nine studies (809 PD patients) also confirmed increased serum TNF-α level in PD (Qin et al., 2016).