Fumarase hydratase deficiency is an inherited disorder that primarily affects the nervous system, especially the brain. It is caused by a lack of the enzyme fumarase, or fumarate hydratase. Newborns with fumarase deficiency show severe neurologic abnormalities, including poor feeding, failure to grow or gain weight at a normal rate, poor muscle tone, lethargy, and seizures. Small head size is often seen in affected individuals, along with brain structure abnormalities. Enlarged liver and spleen, pancreatitis, excess red blood cells and/or deficiency of white blood cells have also been reported. Severely affected individuals are non-verbal and non-ambulatory while less severely affected individuals may have only moderate intellectual disability. Many severely affected children die during early childhood. Some less severely affected individuals have lived into adulthood. No effective treatment is currently available. This condition is caused by pathogenic variants in the fumarate hydratase (FH) gene.