A progressive degenerative disorder of the nervous system belonging to a group of conditions called motor system disorders. Parkinson's disease typically begins in middle age with an average age of onset of 59 years, but it can also begin in the 30s or 40s as well as in the 70s or later. Males are affected slightly more commonly than females; all ethnic groups can develop the disease, with Caucasians at somewhat higher risk. Parkinson's disease results in the loss of brain cells that produce dopamine. The cause is unknown, but, increasingly, specific environmental and genetic factors are being evaluated for their role in the disease. Typically Parkinson's disease is sporadic, but it can occur in familial clusters; and research on some of these families has resulted in the discovery of a few genetic mutations. To date, however, genetic testing is not usually performed due to the low chance of finding a genetic cause for an affected individual.