This domain is found in molybdopterin cofactor (Mo-co) oxidoreductases. It is involved in dimer formation, and has an Ig-fold structure [1].

1: Molecular basis of sulfite oxidase deficiency from the structure of sulfite oxidase. Kisker C, Schindelin H, Pacheco A, Wehbi WA, Garrett RM, Rajagopalan KV, Enemark JH, Rees DC; Cell 1997;91:973-983. PMID:9428520