Progressive accumulation of clumps of the protein alpha-synuclein in the brains of patients with Parkinson's disease coincides with the onset of motor dysfunction. However, whether these clumps are sufficient to trigger neurodegeneration, and how these clumps spread throughout the brain, remained unclear until now. A team led by University of Pennsylvania School of Medicine studied mice expressing a mutated form of alpha-synuclein found in patients with Parkinson's disease. These mice show symptoms of disease around one year of age but not earlier. Scientists found that injecting preformed clumps of human alpha-synuclein into the brains of young mice accelerated disease onset and severity. These clumps seemed to act as "seeds" that recruited even the mouse version of alpha-synuclein into new clumps, which then spread throughout the brain. The pattern of spreading from neuron to neuron suggests that the clumps may hijack the highway traveled by normal brain signals.