In human postmortem studies, TG2 mRNA expression, protein level, and activity are increased in the substantial nigra of PD-affected brains, and TG2-mediated ε-(γ-glutamyl) lysine bonds (GGEL) bonds co-localize with α-Syn in Lewy bodies, and the two proteins co-immunoprecipitate in extracts of PD substantia nigra [144,148]. This evidence concerns the gene TGM2 and Parkinson disease.