Heterochromatin is a tightly packed form of DNA or condensed DNA , which comes in many varieties. These strands lie on a continuum between the two extremes of constitutive heterochromatin and alternative heterochromatin . Both play a role in gene expression . Because it is tightly packed, it was considered inaccessible to polymerase and therefore not transferred, although according to Volpe et al. (2002),  and several other papers,  much of this DNA is in fact transcribed, but it can be found through RNA-induced transcriptional silencing .is changed continuously .(RITS). Recent studies with electron microscopy and OsO4 staining suggest that the dense packing is not due to chromatin.
Constitutive heterochromatins can affect nearby genes (eg position-effect changes ). It is usually repetitive and has structural functions such as centromeres or telomeres , in addition to acting as an attractant for other gene-expression or repression signals.
Elective heterochromatins results in genes that are silenced through RNAi through mechanisms such as histone deacetylation or PV-interacting RNA ( piRNA ) . It is not repetitive and shares the compact structure of constitutive heterochromatins. However, under specific developmental or environmental signaling cues, it may lose its condensed structure and become transcriptionally active. 
Heterochromatin has been associated with di- and tri -methylation of H3K9 in parts of the genome.  H3K9me3- related methyltransferases play an important role in modulating heterochromatin during lineage commitment at the beginning of organogenesis and in maintaining lineage fidelity.
Chromatin is found in two varieties: euchromatin and heterochromatin.  Originally, the two forms were distinguished cytologically by how intensely they stain – euchromatin stains less intensely, whereas heterochromatins stains intensely, indicating tighter packing. Heterochromatins is usually localized at the periphery of the nucleus . Despite this initial dichotomy, recent evidence in both animals  and plants  has suggested that there are more than two distinct heterochromatin states, and that it may actually exist in four or five ‘kingdoms’. , each is marked by different combinations. Epigenetic marks.
Heterochromatin consists mainly of genetically inactive satellite sequences ,  and many genes are repressed to various extents, although some may not be expressed in euchromatin at all.  Both the centromere and telomeres are heterochromatic, as is the Barr body of another, inactive X-chromosome in a female.
Heterochromatin is associated with many functions, from gene regulation to protection of chromosome integrity;  Some of these roles may be attributed to the dense packing of DNA, which makes it less accessible to protein factors that normally bind DNA or the factors associated with it. For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged or viral DNA, leading to cell cycle arrest, DNA repair or fragment destruction, as in bacteria by endonucleases.
Some regions of chromatin are very densely packed with fibrils that exhibit a position comparable to that of the chromosome in mitosis. Heterochromatins is usually inherited clonally; When a cell divides, the two daughter cells usually have heterochromatins within the same regions of DNA, resulting in epigenetic inheritance. Variations cause heterochromatins to encroach on adjacent genes or retract from genes at the extremes of the domain. Being located in these boundary regions (in the cis ) may suppress transcriptional material. This gives rise to expression levels that vary from cell to cell,  which can be demonstrated by position-effect changes. Insulator sequences can act as a barrier in the rare cases where constitutive heterochromatin and highly active genes are intertwined (for example the 5’Hs4 insulator upstream of the chicken β-globin locus,  and Two loci in Saccharomyces spp.
All cells of a particular species package similar regions of DNA into constitutive heterochromatin, and thus in all cells, any genes contained within constitutive heterochromatin will be poorly expressed. For example, all human chromosomes 1, 9, 16, and the Y-chromosome contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near the telomeres.
Regions of DNA packaged in facultative heterochromatin will not be consistent between cell types within a species, and thus sequences in a cell that are packaged in facultative heterochromatin (and the genes within are poorly expressed) can be packed into euchromatin. (And the genes within are no longer silenced). However, the formation of alternative heterochromatin is regulated, and is often associated with morphogenesis or differentiation. An example of alternative heterochromatin is X chromosome inactivation in female mammals: one X chromosome is packaged as alternative heterochromatin and silenced, while the other X chromosome is packaged as euchromatin and expressed .
Among the molecular components controlling the proliferation of heterochromatin are Polycomb-group proteins and non-coding genes such as Xist. The mechanism of such diffusion is still a matter of controversy.  The Polycomb repressive complexes PRC1 and PRC2 regulate chromatin compaction and gene expression and play a fundamental role in developmental processes. PRC-mediated epigenetic aberrations are associated with genome instability and malignancy and play a role in the fidelity of the DNA damage response, DNA repair and replication.
Saccharomyces cerevisiae , or budding yeast, is a model eukaryote and its heterochromatin is well defined. Although most of its genome can be described as euchromatin, S. cerevisiae has regions of DNA that are very poorly transcribed. These loci are the so-called silent mating type loci (hml and hmr), rDNA (encoding ribosomal RNA), and sub-telomeric regions. Fission yeast ( Schizosaccharomyces pombe ) uses another mechanism for heterochromatin formation at its centromere. Gene silencing at this locus depends on components of the RNAi pathway. Double-stranded RNA is believed to silence the region through a series of steps.
In the fission yeast Schizosaccharomyces pombe , two RNAi complexes, the RITS complex and the RNA-directed RNA polymerase complex (RDRC), are part of the RNAi machinery involved in the initiation, proliferation and maintenance of heterochromatin assembly. These two complexes localize in an siRNA-dependent manner on chromosomes at the site of heterochromatin assembly. RNA polymerase II synthesizes a transcript that serves as a platform for the recruitment of RITS, RDRCs and possibly other complexes required for heterochromatin assembly.   Both RNAi and an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These mechanisms of Schizosaccharomyces pombe may occur in other eukaryotes. A large RNA structure called revasin has also been implicated in the production of siRNA to mediate heterochromatin formation in some fission yeast.