SLC25A13 and citrullinemia type I: Although mammals have a specific mitochondrial aspartate exporter, UCP2 [14,15,16,17], it seems that the cytosolic aspartate required for the synthesis of argininosuccinate is provided by the exchange reaction catalyzed by AGC2, the liver-specific isoform, since mutations in SLC25A13 cause an adult form of citrullinemia (CTLN2) in humans [7,18].