Editing Histone (section)

==== Bivalent promoters ====

Analysis of histone modifications in embryonic stem cells (and other stem cells) revealed many gene promoters carrying both [[#Actively transcribed genes|H3K4Me3]] and [[#Repressed genes|H3K27Me3]], in other words these promoters display both activating and repressing marks simultaneously. This peculiar combination of modifications marks genes that are poised for transcription; they are not required in stem cells, but are rapidly required after differentiation into some lineages. Once the cell starts to differentiate, these bivalent promoters are resolved to either active or repressive states depending on the chosen lineage.<ref>{{cite journal | vauthors = Bernstein BE, Mikkelsen TS, Xie X, Kamal M, Huebert DJ, Cuff J, Fry B, Meissner A, Wernig M, Plath K, Jaenisch R, Wagschal A, Feil R, Schreiber SL, Lander ES | title = A bivalent chromatin structure marks key developmental genes in embryonic stem cells | journal = Cell | volume = 125 | issue = 2 | pages = 315–26 | date = April 2006 | pmid = 16630819 | doi = 10.1016/j.cell.2006.02.041 | doi-access = free }}</ref>