Defects in two mitochondrial transporters also lead to UCD: citrin deficiency (mitochondrial aspartate/glutamate carrier deficiency; OMIM #605814) and mitochondrial ornithine transporter 1 (ORNT1) deficiency (ORNT1D), also referred to as hyperornithinemia‐hyperammonemia‐homocitrullinuria (HHH) syndrome (OMIM #238970). This evidence concerns the gene SLC25A15 and urea cycle disorder.