heterochromatin

Heterochromatin is a tightly packed form of chromatin, which comes in multiple varieties. These varieties lie on a continuum between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role in the expression of genes, and correlate with late replication timing. Importantly, heterochromatin was once thought to be tightly coupled with structural compactness, phase separation and also deterministic transcriptional silencing. However, these notions have been increasingly challenged in recent years.

Many epigenetic marks are associated with heterochromatin. The two major ones are H3K27me3 and H3K9me2/me3. Other heterochromatin-associated marks include H4K20me3, H3K56me3, and H3K64me3. Canonically, H3K27me3 is more often associated with facultative heterochromatin, whereas H3K9 is more often associated with constitutive heterochromatin. In many current naming conventions, the former is referred to as "Polycomb" because of its interactions with Polycomb family proteins, whereas the latter is referred to more simply as "heterochromatin."