These belong to an exclusive class of neuromuscular disorders and they all involve a defect in the mitochondrial electron transport chain with devastating outcome. These are maternally transmitted genetic disorders with a mutation in the mitochondrial DNA, leading to in a severe disruption in the energy-producing process. One or more of the CoQ10-dependent enzyme systems may be affected and these respond well to high dose oral CoQ10 therapy. Also, in human myotonic dystrophic conditions, a strong correlation is seen with defects in mitochondrial function and oxidative damage, and significant clinical improvement has been observed with CoQ10 administration.