Xanthine dehydrogenases, that also bind FAD/NAD, have essentially no similarity.

1: The sequence of squash NADH:nitrate reductase and its relationship to the sequences of other flavoprotein oxidoreductases. A family of flavoprotein pyridine nucleotide cytochrome reductases. Hyde GE, Crawford NM, Campbell W; J Biol Chem 1991;266:23542-23547. PMID:1748631