An important type of enzyme that performs redox reactions in the cell using NAD or NADH as a cofactor. Most neccesarily contain a rossman fold to bind the dinucleotide as one domain, with the substrate binding region as another domain. Essentially, this is modular design of a protein with the two functions (cofactor binding and substrate binding) as separate evolutionary units. Since DHs are essential to most metabolic pathways, including glycolysis and the TCA cycle, it is possible that they are quite ancient proteins.