MOCS2 encodes two protein subunits (MOCS2A and MOCS2B) through a bicistronic transcript with overlapping reading frames, functioning as essential components of the molybdopterin synthase complex 12. This complex catalyzes the conversion of precursor Z into molybdopterin by mediating the incorporation of two sulfur atoms to generate a dithiolene group, which is crucial for molybdenum cofactor (MoCo) biosynthesis 1. MOCS2A serves as a sulfur donor after being thiocarboxylated at its C-terminus, while MOCS2B transfers the sulfur to precursor Z to form molybdopterin 1. MoCo is required for the activity of molybdoenzymes including sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase 3. Pathogenic variants in MOCS2 cause molybdenum cofactor deficiency type B, characterized by neonatal-onset seizures, feeding difficulties, delayed motor development, and progressive neurological damage 43. The disease typically leads to early childhood death due to sulfite toxicity from deficient sulfite oxidase activity 56. Unlike type A deficiency, no effective substitution therapy currently exists for MOCS2-related deficiency, though rare mild cases with later onset and better prognosis have been reported 7.