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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).
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