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.