The Heterochromatin reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Heterochromatin

Heterochromatin: Chromosomal material that is darkly staining and tightly coiled throughout the cell cycle and that is, for the most part, genetically inactive. There are two types of heterochromatin: constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin is fixed and irreversible in form and function. Chromosomes 1, 9, 16 and the Y chromosome contain regions of constitutive heterochromatin. Facultative heterochromatin has the faculty to return to the normal euchromatic state. The inactive X chromosome is made up of facultative heterochromatin

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