is an inherited condition that primarily affects the nervous system, especially the brain. Affected infants may have an abnormally small head size (microcephaly), abnormal brain structure, severe developmental delay, weak muscle tone (hypotonia), failure to thrive, seizures, and unusual facial features. Others may have an enlarged liver and spleen (hepatosplenomegaly), an excess of red blood cells (polycythemia), and/or or deficiency of white blood cells (leukopenia) in infancy. Affected individuals usually survive only a few months, but a few have lived into early adulthood. This condition is caused by mutations in the FH gene and is inherited in an autosomal recessive fashion. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.