The coevolution of bacterial pathogens and their hosts has contributed to the development of very complex and sophisticated functional pathogen--host interfaces. Thus, well-adapted pathogens have evolved a variety of strategies to manipulate host cell functions precisely. For example, a group of unrelated Gram-negative pathogenic bacteria have evolved a toxin, known as cytolethal distending toxin (CDT), that has the ability to control cell cycle progression in eukaryotic cells. Recent studies have identified CdtB as the active subunit of the CDT holotoxin. Through its nuclease activity, CdtB causes limited DNA damage, thereby triggering the DNA-damage response that ultimately results in the observed arrest of the cell cycle. In addition, it has been established that CDT is a tripartite AB toxin in which CdtB is the active 'A' subunit and CdtA and CdtC constitute the heterodimeric 'B' subunit required for the delivery of CdtB into the target cell. The mechanism of action of CDT suggests that the infliction of limited damage could be a strategy used by pathogenic bacteria to modulate host cell functions.