In pluripotent ES cells, key developmental regulators contain `bivalent chromatin domains' – regions that carry epigenetic markers of both repressed and active chromatin, and that assemble RNA polymerase (RNAP) complexes. Thus, these bivalent domains silence genes, but keep them primed for timely activation and are thought to resolve into repressed or active domains upon ES cell differentiation. But are bivalent chromatin domains involved in in vivo development? On p. 2483, Véronique Azuara and colleagues report that these domains operate in the early mouse embryo. They show that several somatic lineage regulators (including Hox factors) retain bivalent chromatin domains in cells that are committed to the extra-embryonic lineage. However, these genes, in contrast to similar genes in pluripotent cells, are not engaged by the Polycomb repressive complex component Ring1B. Instead, these bivalent genes are selectively targeted for Suv39h1-mediated repression through H3K9 methylation, and for RNAP exclusion upon trophoblast lineage commitment. Thus, Ring1B and Suv39h1 play mutually exclusive roles in the establishment of distinct chromatin states during early mouse lineage commitment.

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