Distribution of the histone modification marks and RNA Polymerase II binding sites relative to transcriptional start sites (TSS) for RefSeq genes. Genes were divided into four categories based on digital gene expression (RPKM values) from high to very low. Sequence reads were then graphed relative to the TSS for each group. Density traces are labeled in blue for genes expressed at the highest levels, red for medium levels, green for low levels and purple for very low levels (see panel insert). Results are shown for the PANC-1 cell line (panels (A) for H3K27me3, (C) for H3K4me3, (E) for H3K4me1 and (G) for RNA Pol II) and for the hTERT-HPNE cell line (panels (B) for H3K27me3, (D) for H3K4me3, (F) for H3K4me1, and (H) for RNA Pol II). Note the “dip” in H3K27me3 sequence tags around the TSS of highly expressed genes in PANC-1 cells.