Polycomb group genes in stem cell self-renewal: a double-edged sword.

Polycomb group (PcG) genes encode chromatin associated proteins that usually form polycomb repressive complexes (PRC) to maintain the repressive state of gene transcription. In both embryonic and adult stem cells, PRCs are commonly regarded as essential players for maintaining stem cell multipotency by repressing developmental genes. However, emerging evidence also points out essential roles of PcG genes in antagonizing stem cell self-renewal and facilitating cell lineage differentiation. Here, we briefly review recent literature on these two seemingly opposite functions of PcG genes in stem cells and discuss future perspective towards understanding polycomb function in stem cells and tumorigenesis.