DNA is the central repository of genetic information in the cell, yet it is under constant attack by chemical mutagens, radiation and other processes. Cells therefore put a great deal of resources into repairing any damage to this precious store. Mechanisms of DNA repair vary greatly in their level of complexity, from specific reversal mechanisms that involve a single protein, to sequential pathways that require many enzymes. But at the heart of all these mechanisms lie proteins that recognize damage to DNA, raising important questions about how damaged DNA may be distinguished. These...