Superposition of five dehydrogenase enzymes having the Rossmann fold (Ghosh et al., Structure, 3, 503, 1995; Varughese et al., Proc. Natl Acad. Sci., USA, 89, 6080, 1992; Tanaka et al., Structure, 4, 33, 1996; Tanaka et al., Acta Cryst. D, in press). Although these five enzymes have twelve fully conserved residues, the entire family of fifty enzymes has only five conserved residues, including the catalytic Tyr156 and Lys159 at the interface between binding sites for the co-factor and substrate. Remarkably, the Rossmann fold persists with almost no conservation of amino acid residues.