MOCOS (molybdenum cofactor sulfurase) is an essential enzyme that sulfurates the molybdenum cofactor, a critical process for the activity of xanthine dehydrogenase (XDH) and aldehyde oxidase enzymes 1. The enzyme facilitates sulfation of molybdenum, enabling the cofactor to be liganded by one oxygen and one sulfur atom in its active form 1. MOCOS plays a central role in purine catabolism, where it works coordinately with XDH in the purine degradation pathway 2. Beyond purine metabolism, MOCOS contributes to cellular redox homeostasis and appears important for proper neurodevelopment and synaptic function 3. Pathogenic variants in MOCOS cause xanthinuria type II, a rare autosomal recessive disorder characterized by extremely low serum uric acid levels, hypoxanthine and xanthine accumulation, and predisposition to xanthine kidney stones 4. The enzyme has been implicated in autism spectrum disorders, where downregulation of MOCOS expression in olfactory stem cells correlates with neurotransmission defects and increased oxidative stress sensitivity 3. Additionally, MOCOS variants show associations with psychiatric disorders through sex-dependent genetic effects 5. The gene's disruption can lead to nucleotide imbalances, replication fork stalling, and compromised antioxidant responses 2.