Aenylosuccinate lyase deficiency is a heritable deficiency which is characterized by the appearance of succinylaminoimidazolecarboxamide riboside (SAICAriboside) and succinyladenosine (S-Ado) in cerebrospinal fluid, urine, and to a lesser extent in plasma. These two succinylpurines are the dephosphorylated derivatives of SAICAribotide (SAICAR) and adenylosuccinate (S-AMP), the two substrates of adenylosuccinate lyase (ASL), which catalyzes an important reaction in the de novo pathway of purine biosynthesis. ASL catalyzes two distinct reactions in the synthesis of purine nucleotides, both of which involve the β-elimination of fumarate to produce either aminoimidazole carboxamide ribotide (AICAR) from SAICAR or adenosine monophosphate (AMP) from S-AMP.