alpha-Synuclein is a Parkinsons-disease-related protein. It forms aggregates in vivo, and these aggregates cause cell cytotoxicity. Aggregation inhibitors are expected to reduce alpha-synuclein cytotoxicity, and an aggregation accelerator has recently been reported to reduce alpha-synuclein cytotoxicity. Therefore, amyloid aggregation modulating ligands are expected to serve as therapeutic medicines.