Histone-modification patterns associated with Pol II stalling and escape. Histone modifications typical of (a) genes with stalled Pol II and (b) after Pol II escape to transcription elongation. Stalled Pol II signals are associated with active histone-modification marks, including histone H3 trimethylation on lysine 4 (H3K4me3) and acetylation of lysine 9 and 14 (H3K9ac, H3K14ac) . For transcript elongation to proceed, not only the histone modification marks mentioned above are necessary; additional histone modifications, including H3 trimethylation on K36 (H3K36me3) and dimethylation on K79 (H3K79me2) are also needed . The colored bars indicate the location of the histone modifications on the transcripts.