Background: Molybdenum cofactor deficiency (MoCD) is characterized by severe and rapidly progressive neurological damage caused by the loss of sulfite oxidase activity, one out of four molybdenum-dependent enzymes. Without effective therapy death in early infancy has been the usual outcome. Objectives: MoCD type A patients carry mutations in MOCS1 causing a loss of cyclic pyranopterin monophosphate (cPMP), the first intermediate in the Moco pathway. An experimental substitution therapy with cPMP has been initiated in six patients. Patients: Patients have been diagnosed in utero or at the age of 2-20 days and treatment was initiated on days 0-36 with daily i.v. administration of 80 μg/kg and stepwise increase to 240 μg/kg body weight. Results: Within days, all urinary markers of sulfite oxidase (sulfite, S-sulfocysteine, thiosulfate) and xanthine oxidase deficiency (xanthine, uric acid) returned to almost normal and stayed constant throughout treatment. Clinically, all infants became more alert, convulsions and twitching disappeared within the first two weeks and the EEG showed the return of rhythmic elements and markedly reduced epileptiform discharges. Conclusion: Substitution of cPMP represents the first causative therapy available for MoCD patients. We demonstrate efficient uptake of cPMP and restoration of molybdenum cofactor-dependent enzyme activities. Further neurodegeneration was stopped. Conflict of Interest declared. We report on the efficacy of long-term cPMP substitution in two unrelated children with MOCD group A who both share the same homozygous mutation in the MOCS1 gene. Diagnoses were made on day 1 of life in Baby 1 due to a previously affected sibling and on day 4 in Baby 2 who started having intractable seizures from day 1 of life. Daily intravenous cPMP infusions were started on day 7 and 5 of life and continued for a period of 18 and 15 months in Baby 1 and 2, respectively, without interruption and using an identical protocol. Treatment efficacy and safety were carefully monitored. Both infants have been tolerating cPMP without adverse effects, apart from intercurrent central venous line infections. We observed a rapid and sustained good clinical and biochemical response. Baby 1 has shown very satisfying developmental progress whereas Baby 2, despite earlier treatment, suffers from static, dystonic cerebral palsy and cystic encephalopathy due to early non-progressive necrotic changes, probably pre-dating cPMP substitution. Infants with MOCD type A can be safely and effectively treated with longterm cPMP substitution. The extent of encephalopathy does not only depend on genotype and time delay between birth and start of cPMP substitution. Alpha-aminoadipic semialdehyde (α-AASA) is the hallmark accumulating metabolite in pyridoxine-dependent epilepsy due to Antiquitin deficiency (PDE). Elevated levels may be measured in urine by LC-MS for diagnosis of the disorder, according to age-dependent reference ranges. The typical ranges seen in Antiquitin deficiency are 7-138 mmol/mol Cr under 12 months and 1.3-40 mmol/mol Cr over 12 months. Patients with molybdenum cofactor deficiency (MoCoF) and PDE may both present with a neonatal epileptic encephalopathy (NEE). Urine samples analysed by LC-MS in our laboratory to aid differential diagnosis of NEE have shown elevated α-AASA in all cases subsequently confirmed by molecular genetic analysis to have MoCoF deficiency. α-AASA ranged from 9.6-16.9 mmol/mol Cr in two patients under 6 months (reference range <5) and 2.9 mmol/mol Cr in a single patient over 12 months of age (reference range <2). α-AASA may form a sulphonate with sulphite when present in excess. This may be excreted in urine and liberated during the alkalinisation step of sample analysis, providing a possible mechanism for the observation. Detection of elevated urinary sulphite or sulphocysteine in MoCoF deficiency will differentiate the two conditions guiding appropriate management. Background: Molybdenum cofactor deficiency (MoCD) is a rare inherited metabolic disorder characterized by severe and progressive neurologic damage mainly caused by resulting sulfite oxidase deficiency (SOD). Elevated urinary levels of sulfite, thiosulfate and S-sulfocysteine (SSC) are hallmarks in the diagnosis of MoCD and SOD.