Protein domain architectures and possible evolutionary trajectories for the constituent domains of the Ku proteins. The domains are indicated by different shapes; the two distinct forms of the HEH are indicated by differential coloring. For each domain architecture, the phyletic distribution is shown in parentheses; the species name abbreviations are as in Figures 2 and 3. The arrows indicate probable evolutionary events such as derivation of a new form of a particular domain (SAP from ancestral HEH) and domain fusion. The connection shown between prokaryotic and eukaryotic forms of the Ku protein does not differentiate between two evolutionary scenarios discussed in the text.