USP9X and Parkinson disease: In the brain tissues of PD patients, USP9X colocalises with α-synuclein inclusions, and in vitro studies show a functional interaction; whilst monoubiquitylated α-synuclein is degraded by the proteasome, USP9X deubiquitylation of α-synuclein directs its degradation by the less efficient autophagy pathway [86].