Although there are many theories about the cause of Parkinson's disease, none has ever been proved. Researchers have reported families with apparently inherited Parkinson's for more than a century. However, until recently, the prevailing theory held that one or more environmental factors caused the disease. Severe Parkinson's-like symptoms have been described in people who took an illegal drug contaminated with the chemical MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and in people who contracted a particularly severe form of influenza during an epidemic in the early 1900s. Recent studies of twins and families with Parkinson's have suggested that some people have an inherited susceptibility to the disease that may be influenced by environmental factors. The strong familial inheritance of the chromosome 4 gene is the first evidence that a gene alteration alone may
lead to Parkinson's disease in some people.