Complex sentence: Many promoters in embryonic stem cells and imprinted genes are associated with both H3K4me3 (activating) and H3K27me3 (repressive) marks, as they allow a gene to be activated or repressed when needed.
Split into simpler sentences: Many promoters in both embryonic stem (ES) cells and imprinted genes are associated with both H3K4me3 (activating) and H3K27me3 (repressive) marks. They allow a gene to be activated or repressed when needed.