Distribution of histone modification marks and RNA Polymerase II binding sites according to gene expression levels for RefSeq genes. Genes were divided into four categories according to gene expression values in RPKM from high (A), medium (B), low (C) to very low (D). Density traces for histone modification sequence tags were then graphed according to the start of transcription (TSS) for each of the four groups. Density traces are labeled in green for H3K4me1, red for H3K4me3, blue for H3K27me3 and purple for RNA Pol II (see panel insert). Results are shown for the PANC-1 cell line; similar results were seen for the hTERT-HPNE cell line (Additional file 9: Figure S3).