The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD⁺, is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.