Researchers studied 650 patients between ages 63-91, including 200 healthy patients and 450 with Parkinson's disease. They analyzed DNA markers for three polymorphisms of nitric oxide (NO) synthase enzymes (eNOS, nNOS, iNOS) that some research linked to loss of dopaminergic neurons in the substantia nigra pars compacta. The researchers found no association between these NO polymorphisms and Parkinson's disease.