Skip to main content

Diseases Studied

The Rare Diseases Clinical Research Network is an NIH-funded research network of 20 active consortia or research groups working to advance treatment for diseases that are rare. Use the search tools on this page to find the diseases we currently study. You can reach out to the indicated consortia or research groups for more information on those diseases and studies underway.

This network focuses on clinical research and does not generally support clinical care outside of research activities. To learn about other rare diseases, please visit the Genetic and Rare Diseases Information Center (GARD), which is an NIH program that helps the public find reliable information about rare and genetic diseases. Their staff are specialists. Contact them at 1-888-205-2311 or email

All Diseases > Complex II deficiency

Complex II deficiency

Alternative Names: Succinate Dehydrogenase Deficiency (SDH Deficiency); Succinate CoQ Reductase Deficiency; Mitochondrial Respiratory Chain Complex II Deficiency; complex 2 Mitochondrial Respiratory Chain Deficiency

Disease Category: Mitochondrial Disorders

An inherited, multisystemic, mitochondrial disorder characterized by protein complex II deficiency. Shortage of this protein complex (structure made up of multiple proteins) disrupts the second step of oxidative phosphorylation, the process by which mitochondria (specialized cell structures) produce energy which the body can use. Typical manifestations include Leigh syndrome, leukodystrophy, which is the destruction of white matter made up of myelin (a fatty coating) around nerves in the brain, as well as seizures, psychomotor regression (loss of mental and motor skills), vision and hearing loss, speech problems, hypertonia (increased muscle tone), extremity muscle atrophy (wasting), hyperreflexia (exaggerated reflexes), cardiomyopathy (heart disease), and fatigue.

Research groups studying this disease

Mitochondrial Disorders
NAMDC logo

North American Mitochondrial Disease Consortium (NAMDC)

Improves quality of life for people affected by mitochondrial disease through education, outreach, advocacy, and clinical research initiatives.

Promotes research and education for the diagnosis, treatment, and cure of mitochondrial disorders while providing support to those affected.