A hypothetical evolutionary scenario for the UDG superfamily. The different families are shown in different colors and potential order and lineage of derivation is indicated on the standard phylogenetic model for the three domains of life. The representation of the active-site pocket residues typical of that set is shown next to each class at the point of derivation. The first position is the general base represented by an aspartate in the UNGs, the second position is the uracil/cytosine discrimination site occurring after the core strand 2, and the third position is typically represented by a histidine that acts as an electrophile. The X at a given position denotes lack of conservation. In some of the AUDGs and the DRUDGs, a glutamate could function as alternative general base.