Domain architecture of the UmuC/DinB superfamily. N, N-terminal nucleotidyl transferase domain; C, conserved C-terminal domain of unknown function; HhH, helix–hairpin–helix; C2HC, Zn cluster; C2H2, Zn finger; X1/X2, uncharacterized domains. The RING domain in Rad18 is designated by a yellow circle. The sequences are grouped by the distinct domain organizations of the respective proteins. Abbreviations: Sc, S. cerevisiae; Sp, S. pombe; Ce, C. elegans; Hs, H. sapiens; Ec, E. coli; 100 AA, 100 amino acids.