DDC and hyperprolinemia type 1: In this study, we identified six genes (DMGDH, AGA, ACY1, PRODH, DDC, CPS1) that have been previously implicated in recessive metabolic disorders, four of which show direct relationships to the amino acids identified here: mutations in AGA are known to cause aspartylglucosaminuria (MIM 208400); mutations in DMGDH cause dimethylglycine dehydrogenase deficiency (MIM 605850); mutations in ACY1 cause aminoacylase-1 deficiency (MIM 609924); and mutations in PRODH are known to cause hyperprolinemia type I (MIM 239500).