MOCS3 (molybdenum cofactor synthesis 3) is a dual-function protein essential for two critical cellular processes: tRNA thiolation and molybdenum cofactor (Moco) biosynthesis 1. The protein contains an N-terminal adenylation domain and a C-terminal rhodanese-like domain that work together to activate sulfur carrier proteins URM1 and MOCS2A through adenylation and subsequent persulfide sulfur transfer, generating thiocarboxylated C-termini 1. In tRNA modification, MOCS3 catalyzes 2-thiolation of mcm(5)S(2)U at wobble positions in cytosolic tRNAs 1. In Moco biosynthesis, MOCS3 mediates thiocarboxylation of MOCS2A, the small molybdopterin synthase subunit, which is critical for the three-step Moco synthesis pathway 2. Unlike bacterial homologs, eukaryotic MOCS3 does not use thiosulfate as its primary sulfur donor; instead, NFS1 provides the persulfide sulfur 3. Loss of MOCS3 function contributes to molybdenum cofactor deficiency, a severe neonatal disorder causing myoclonic seizures and encephalopathy due to sulfite oxidase dysfunction 4. Additionally, MOCS3 variants have been associated with chr20 obstructive pulmonary disease susceptibility 5, and the gene has been implicated in oncogene-induced replication stress in cancer 6.