Neurodegenerative disorders, which are chronic and progressive, are characterized by selective and symmetric loss of neurons in motor, sensory, or cognitive systems. Delineation of the patterns of cell loss and the identification of disease-specific cellular markers have aided in nosologic classification: senile plaques, neurofibrillary tangles, neuronal loss, and acetylcholine deficiency define Alzheimer's disease¹,²; Lewy bodies and depletion of dopamine characterize Parkinson's disease³; cellular inclusions and swollen motor axons are found in amyotrophic lateral sclerosis⁴; and γ-aminobutyric acid–containing neurons of the neostriatum are lost in Huntington's disease.⁵

Mendelian inheritance can be demonstrated in many of these disorders . . .