English: Heterochromatin contains specialized proteins (red) that bind to histone H3 or H4 subunits that have been marked by a specific modification (green). The enzyme that performs this modification is also present in heterochromatin (blue), ensuring that the modification is maintained. When the chromosome is duplicated, the marked histone H3–H4 tetramers of the parent chromosome are distributed randomly to the two daughter strands, resulting in a mixture of old (light grey) and new (dark grey) nucleosomes. In heterochromatin, new nucleosomes are rapidly marked by the histone-modifying enzymes bound to old nucleosomes. This provides new binding sites for heterochromatin proteins. These proteins (such as the Sir complex or HP1) also have the ability to bind to each other, further promoting the assembly of a protein polymer along the chromosome. It can also be seen in this diagram that the boundary between heterochromatin and euchromatin is not rigidly fixed, because small local changes in the extent of histone modification could cause shifts in its position.^[1]