The observed decrease in biotinylated 0N3R tau in fibril-treated neurons expressing WT α-synuclein, combined with the absence of biotinylated α-synuclein in the neurons expressing 0N4R tau, suggests that isoform-specific tau–α-synuclein interactions may be involved in neurodegenerative disease progression. The gene discussed is MAPT; the disease is neurodegenerative disease.