The regulatory elements that direct tissue-specific gene expression in the 
developing mammalian embryo remain largely unknown. Although chromatin profiling 
has proven to be a powerful method for mapping regulatory sequences in cultured 
cells, chromatin states characteristic of active developmental enhancers have 
not been directly identified in embryonic tissues. Here we use 
whole-transcriptome analysis coupled with genome-wide profiling of H3K27ac and 
H3K27me3 to map chromatin states and enhancers in mouse embryonic forelimb and 
hindlimb. We show that gene-expression differences between forelimb and 
hindlimb, and between limb and other embryonic cell types, are correlated with 
tissue-specific H3K27ac signatures at promoters and distal sites. Using H3K27ac 
profiles, we identified 28,377 putative enhancers, many of which are likely to 
be limb specific based on strong enrichment near genes highly expressed in the 
limb and comparisons with tissue-specific EP300 sites and known enhancers. We 
describe a chromatin state signature associated with active developmental 
enhancers, defined by high levels of H3K27ac marking, nucleosome displacement, 
hypersensitivity to sonication, and strong depletion of H3K27me3. We also find 
that some developmental enhancers exhibit components of this signature, 
including hypersensitivity, H3K27ac enrichment, and H3K27me3 depletion, at lower 
levels in tissues in which they are not active. Our results establish histone 
modification profiling as a tool for developmental enhancer discovery, and 
suggest that enhancers maintain an open chromatin state in multiple embryonic 
tissues independent of their activity level.