Disordered mitochondrial metabolism may play an important role in a nu
mber of idiopathic neurodegenerative disorders. The question of mitoch
ondrial dysfunction is particularly attractive in the case of idiopath
ic Parkinson disease (PD), since Vyas et al. recognized in the 1980s t
hat the parkinsonism-inducing compound N-methyl-4-phenyl-1,2,3,6-tetra
hydropyridine is a mitochondrial toxin. The unique genetic properties
of mitochondria also make them worthy of consideration for a pathogeni
c role in PD, as well as in other late-onset, sporadic neurodegenerati
ve disorders. Although affected persons occasionally do provide family
histories that suggest Mendelian inheritance, the vast majority of th
e time these diseases appear sporadically. Because of unique features
such as heteroplasmy, replicative segregation, and threshold effects,
mitochondrial inheritance can allow for the apparent sporadic nature o
f these diseases.