In his historic manuscript on the "shaking palsy," James Parkinson expressed the hope "that some remedial process may ere long be discovered, by which, at least, the progress of the disease may be stopped."(1) Realization of that hope will require not only the discovery of a treatment that can halt or slow the progression of Parkinson disease (PD), but also the identification of early, accurate, and accessible biomarkers, for by the time the cardinal clinical signs of bradykinesia, rigidity, tremor, and postural instability appear, substantial destruction of nigrostriatal dopaminergic neurons has already occurred.