These disorders are caused by biallelic mutations in the genes phosphoribosylformylglycineamidine synthase (PFAS), phosphoribosylaminoimidazolecarboxylase/phosphoribosylaminoimidazolesuccinocarboxamide synthase (PAICS), adenylosuccinate lyase (ADSL), and aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC), resulting in the disorders PFAS deficiency, PAICS deficiency, ADSL deficiency, and AICAribosiduria, respectively (Figure 1) [1,2,3,4,5]. The gene discussed is ATIC; the disease is AICA-ribosiduria.