Molybdenum cofactor (MoCo) deficiency leads to a combined deficiency of the molybdoenzymes sulphite oxidase, xanthine dehydrogenase and aldehyde oxidase. Effective therapy is not available for this rare disease, which results in neonatal seizures and other neurological symptoms identical to those of sulphite oxidase deficiency. It is an autosomal recessive trait and leads to early childhood death. Biosynthesis of MoCo can be divided into the formation of a precursor and its subsequent conversion to the organic moiety of MoCo by molybdopterin synthase. These two steps are the molecular basis of the two observed complementation groups A and B and of two types of MoCo deficiency with an identical phenotype. MOCS1 is defective in the majority of patients (group A) and was shown to encode two enzymes functioning in the formation of a precursor. The corresponding transcript is bicistronic with two consecutive open reading frames (ORFs). MOCS2 encodes the small and large subunits of molybdopterin synthase via a single transcript with two overlapping reading frames. This gene carries lesions in the B complementation group less frequently observed in patients. Both genes, MOCS1 and MOCS2, share the unusual bicistronic architecture, have identical and very low expression profiles and extremely conserved C-terminal ends in their 5'-ORF. These observations point to a novel form of microcompartmentalization and render the MOCS genes ideal candidates for a somatic gene therapy approach.