TGM2 and Parkinson disease: In human studies, compared with control subjects, a significant increase in TG2 protein and mRNA expression is found in the substantia nigra of PD patients (Citron et al., 2002; Andringa et al., 2004; Wilhelmus et al., 2011) as well as increased TG2 protein levels in their cerebrospinal fluid (Vermes et al., 2004).