Heterochromatin—many flavours, common themes

Heterochromatin remains condensed throughout the cell cycle, is generally transcriptionally inert and is built and maintained by groups of factors with each group member sharing a similar function. In mammals, these groups include sequence-specific transcriptional repressors, functional RNA and proteins involved in DNA and histone methylation. Heterochromatin is cemented together via interactions within and between each protein group and is maintained by the cell's replication machinery. It can be constitutive (permanent) or facultative (developmentally regulated) and be any size, from a gene promotor to a whole genome. By studying the formation of facultative heterochromatin, we have gained information about how heterochromatin is assembled. We have discovered that there are many different architectural plans for the building of heterochromatin, leading to a seemingly never-ending variety of heterochromatic loci, with each built according to a general rule.