MAPT and Parkinson disease: The fact that humans express only one functional Skp1 isoform (Semple 2003) combined with the decreased expression in PD SNpc may account for the wide impairment in the function of proteins implicated in DAergic neurotransmission and in the accumulation of a wide spectrum of ubiquitinated protein aggregates in brains of PD patients, such as TH, synphilin-1, α-synuclein, and phosphorylated tau.