Promyelocytic leukemia nuclear bodies (PML NBs) – large multiprotein foci that exist within the nucleus of mammalian cells – are involved in diverse cellular processes that include apoptosis, the DNA-damage response and angiogenesis, but several key questions about the biochemical basis of their function remain. For instance, how dynamic is the protein composition of individual NBs, and what controls the association of component proteins? To shed light on these questions, Peter Hemmerich and colleagues (p. 2731) conduct a thorough analysis of the intranuclear dynamics of 11 component proteins of PML NBs, including the six splice variants of the PML protein (PML I-VI). Using FRAP, fluorescence correlation spectroscopy and computer modelling, the authors show that the PML isoforms exhibit markedly different residence times at PML NBs; in particular, PML V exchanges unusually slowly and might, the authors propose, act as a scaffold for other component proteins. The other proteins that are analysed – DAXX, BLM, HIPK2 and SP100 – exchange much more rapidly than the PML isoforms. Notably, conjugation by SUMO modulates the rate of exchange of PML isoforms and SP100. PML-NB composition is, therefore, highly plastic and subject to subtle regulation.