Parkinson's Disease is the most common neurodegenerative movement disorder and afflicts more than 1 million persons in the US (with an estimated prevalence of 1% of the population over 50 years). Parkinson's disease is characterized by profound loss of neurons in the substantia nigra. The loss of dopamine neurons in the substantia nigra results in the degeneration of the nigrostriatal dopamine system that regulates motor function. This, in turn, leads to motor dysfunction, consisting of poverty and slowness of voluntary movements, tremor, stooped posture, rigidity, and gait disturbance. There is no cure for Parkinson's disease. Long term medical therapy with levodopa leads to severe secondary motor complications and striatal dysfunction.

The exact cause of Parkinson's disease remains unknown. Lewy bodies are found in pigmented brain stem neurons. 80% of cell loss before clinical symptoms occur. Neural transplantation aims at replacing the loss of nigrostriatal neurons within the striatum itself.