Mobile foci of Sp100 do not contain PML: PML bodies are immobile but PML and Sp100 proteins are not. 
PML bodies are nuclear organelles that are associated with various diseases and are suggested to be involved in multiple cellular activities including transcriptional regulation, apoptosis, and antiviral defence.  Because many proteins with different functions aggregate in PML bodies, it has also been suggested that these bodies function as nuclear depots.  Some proteins consistently found in PML bodies may form a stable scaffold that regulates the recruitment of other proteins.  Thus, some proteins might be stably integrated into PML bodies while others continuously exchange with the nucleoplasm.  To study the dynamic properties of PML bodies and resident proteins, we constructed fusion proteins of Sp100, PML, and CBP with autofluorescent proteins.  Using time-lapse imaging, we show that PML bodies exhibit little movement but that small foci that contain Sp100 but not PML are dynamic and fuse with PML bodies.  Furthermore, we show by monitoring fluorescence recovery after photobleaching that Sp100, PML, and CBP are dynamic components of PML bodies.  This suggests that these proteins do not play a strict structural role in these bodies but that they function at other sites in the nucleoplasm.