(Top) Schematic of the A–U–M model. A given histone has an activating (A) or a silencing mark (M) or is unmodified (U). The curved arrows represent self-reinforcing interactions where M histones promote transitions towards M (red arrows) and A histones promote transitions towards A (green arrows). (Bottom) The A–U–M model generates bistable states with histones predominantly either A (ON) or M (OFF). The states are maintained through replication despite the incorporation of new unmodified U histones at DNA replication. The self-reinforcing feedbacks can add marks to neighbouring histones (short-ranged, solid arrow) or, in addition, to distant histones (long-ranged, dashed arrow), thereby rebuilding the digital state that existed prior to replication.