Heterochromatin is the name given to regions of chromosomes that are packaged in such a way as to be transcriptionally inactive. They are condensed so that the proteins involved in transcription, such as RNA polymerase, cannot access the DNA sequence that would be transcribed. For instance, histones may be deacetylated so as to strengthen their association with DNA.

Heterochromatin comes in two flavours: constitutive heterochromatin remains transcriptionally silent for good, while facultative heterochromatin may revert to transcriptionally active euchromatin at certain times.