In the cases of ultra-rare diseases, medical charts have been used as the basis for drug approval, e.g. CARBAGLU (Carglumic Acid) for the treatment of the deficiency of the hepatic enzyme N-acetylglutamate synthase (NAGS), the rarest of the Urea Cycle Disorders (UCDs), affecting fewer than 10 patients in the U.S. at any given time and fewer than 50 patients worldwide. Here, NAGS is linked to urea cycle disorder.