Histone modifications at the promoters of key developmental regulator genes in embryonic stem (ES), trophectoderm stem (TS) and extra-embryonic endoderm (XEN) cells. The abundance of active [histone 3 lysine 4 dimethylation (H3K4me2, yellow bars), histone 4 acetylation (H4ac, blue bars), histone 3 lysine 9 acetylation (H3K9ac, white bars)] and repressive [histone 3 lysine 27 trimethylation (H3K27me3, purple bars), histone 4 lysine 20 trimethylation (H4K20me3, red bars)] histone marks at selected loci was assessed in ES, TS and XEN cells by chromatin immunoprecipitation and quantitative polymerase chain reaction. Values are shown as the ratio of modified histone H3 to unmodified histone H3 immunoprecipitations and normalized to an abundantly expressed gene in each cell type; Oct4 in ES cells, Cdx2 in TS cells and Gata6 in XEN cells. Detected transcripts are highlighted in green while overt gene expression is shown in bold green. Primers were designed to the promoter region (100-600 bp upstream the transcriptional start site). Error bars represent the standard deviation of three independent experiments.