Emerging roles of Polycomb silencing in X-inactivation and stem cell maintenance

Maintenance of cell identity and cell fate depends on the tight regulation of gene expression patterns in correct time and space. Two families of proteins, the trithorax group (trxG) and the Polycomb group (PcG), use epigenetic mechanisms to faithfully ensure that designated genes are maintained on or off throughout the life of the organism, This maintenance function is imperative to allow the proper development of an organism from a single cell to an organized combination of multifunctional cells. Here, we briefly review the advances achieved in recent years aimed at understanding how members of PcG and trxG function (for more in-depth reviews, see Otte and Kwaks 2003; Pirrotta et al. 2003; Lund and van Lohuizen 2004; Valk-Lingbeek et al. 2004). Particularly, we will discuss methods that can be employed to uncover additional target genes regulated by PcG and/or trxG families. Additionally, we will focus on recent results linking PcG regulation with X-inactivation and with stem cell biology