Polycomb group (PcG) proteins play important roles in maintaining the repressed transcriptional state of genes. PcG proteins operate as part of Polycomb repressive complexes (PRCs). 'Core' PRCs have been purified that contain only a limited number of PcG proteins. In addition, many gene regulatory proteins have been identified to interact with PcG proteins. However, it remains subject to discussion whether these interactions are transient or whether the regulatory proteins are real components of PRCs. It has also become clear that the compositions of 'core' PRCs differ amongst cell types and that extensive changes in compositions occur during the embryonic development of cells. Because of these dynamic changes, we argue that speaking of a definitive core PRC can be misleading.