Fig. 1. Polycomb and Trithorax in stem cells and differentiated cells. (A) Stem cells have a high capacity to proliferate and to generate different differentiated cell types, and following division can give rise to a new stem cell and a differentiated daughter cell. (B) Classes of genes that must be active or silenced in stem cells and differentiated cells are shown. (Left) Tumor suppressors and genes specifying cell fate are silenced in stem cells, whilst genes conferring `stemness' are active. (Right) The activities of tumor suppressors and `stemness' genes are reversed in differentiated cells, which have limited proliferation capacity. Most genes that specify different cell fates continue to be silenced in differentiated cells, except for those that are required to specify a given fate. The PcG proteins target many genes of the three classes shown and are essential in stem cells and differentiated cells, both for the maintenance of silent or active states and for the switching of these states upon differentiation (see main text for details).