In order to detect tau–tau interactions in situ, we took advantage of the knowledge we generated developing the alpha-synuclein proximity ligation assay for Parkinson’s disease [53], namely, the ability of PLA to detect protein interactions of two or more molecules, but not monomers, when conjugating the same epitope blocking monoclonal antibody to PLUS and MINUS PLA probes. The gene discussed is SNCA; the disease is Parkinson disease.