Figure 1.

Bivalent domains mark CpG-rich promoters of developmental genes in ES cells. Whereas promoters of housekeeping genes such as Polm are marked solely with H3K4me3, bivalent promoters carry both H3K4me3 and H3K27me3. The latter genes either are expressed at low levels or are silent in ES cells but may become activated upon differentiation to neural precursor cells (NPCs) or be expressed in cells of reduced differentiation potential, such as MEFs. Conversely, other genes may become silent upon differentiation. Promoters with low CpG content may be activated during differentiation by other means, as they lack H3K4me3 and H3K27me3 in ES cells, and their regulation is likely distinct from CpG-rich promoters. The ChIP-seq traces are based on data from Mikkelsen et al. (2007).