Abstract
Heterochromatin remains condensed throughout the cell
cycle, is generally transcriptionally inert and is built and
maintainedbygroupsoffactors witheachgroupmember
sharing a similar function. In mammals, these groups
include sequence-specific transcriptional repressors,
functionalRNAandproteinsinvolvedinDNAandhistone
methylation. Heterochromatin is cemented together via
interactions within and between each protein group and
ismaintainedbythecell’sreplicationmachinery.Itcanbe
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