S100A9 and Parkinson disease: Remarkably, S100A9 protofilaments and especially its annular species closely resemble similar amyloid structures of leading amyloid polypeptides such as Aβ and α-synuclein in AD and Parkinson’s disease, respectively (Fig. 5, [24]) and they are also cytotoxic towards SH-SY5Y neuroblastoma cells (Fig. 5).