<p>Members of this protein are the small subunit (or, in eukaryotes, the N-terminal domain) of xanthine dehydrogenase, an enzyme of purine catabolism via urate. The small subunit contains both an FAD and a 2Fe-2S cofactor. Aldehyde oxidase (retinal oxidase) appears to have arisen as a neofunctionalisation among xanthine dehydrogenases in eukaryotes. </p>