Background During the past 10 years there has been growing interest in the role of mitochondrial dysfunction in many diseases,including heart disease,metabolic disorders,neurological diseases,and nutritional disorders.While biochemical analysis of electron transport complex (ETC) enzymes has been accepted as a cornerstone for assessment of mitochondrial disease,clinical use of ETC testing is somewhat limited.Coenzyme Q 10 (CoQ 10),an essential mobile electron carrier of the mitochondrial respiratory chain,has received attention because CoQ 10 tissue content varies in response to stress and disease.Muscle CoQ 10 deficiency,which may be primary or secondary,was once thought to be rare,but new reports suggest that CoQ 10 deficiency may be much more common than previously thought.Also there is interest because recent data suggest that CoQ 10 deficiency may predict a patient s responsiveness to supplementation with CoQ10 and possibly other agents.