The spindle is a dynamic, microtubule-based structure responsible for chromosome segregation during cell division. Spindles in mammalian cells contain several thousand microtubules that are arranged into highly symmetric bipolar arrays by the actions of numerous microtubule-associated motor and non-motor proteins. In addition to these protein constituents, recent work has demonstrated that poly(ADP-ribose) is a key spindle component. Of the multitude of poly(ADP-ribose) polymerase proteins encoded in the genome, tankyrase 1 appears to be the primary enzyme responsible for building poly(ADP-ribose) in spindles during mitosis. In this issue of the Biochemical Journal, Susan Smith and co-workers show that the primary target of tankyrase 1 in dividing cells is NuMA (nuclear mitotic apparatus protein), a protein that cross-links microtubule ends at spindle poles. The impact of poly(ADP-ribosyl)ation on the biochemical function of NuMA remains murky at this time, but these new results represent the first step to clearing the view as to how poly(ADP-ribosyl)ation regulates cell division.