Glutamate formiminotransferase deficiency is an autosomal recessive disorder and the second most common inborn error of folate metabolism. Features of a severe phenotype include elevated levels of formiminoglutamate (FIGLU) in the urine in response to histidine administration, megaloblastic anemia, and mental retardation. Features of a mild phenotype include high urinary excretion of FIGLU in the absence of histidine administration, mild developmental delay, and no hematologic abnormalities (summary by Hilton et al., 2003).