Constitutive heterochromatin

Constitutive heterochromatin domains are sections of DNA that occur throughout the chromosomes of eukaryotes, but particularly at the centromeres and telomeres. They often consist of very highly condensed, repetitive DNA and are largely transcriptionally silent. These regions normally appear darkly stained in karyograms.

Unlike facultative heterochromatin, constitutive heterochromatin is never converted back into euchromatin. The regions of DNA that exist as constitutive heterochromatin will be the same for all cells of a given species.