Heterochromatin

Heterochromatin is the darker part of chromatin in the nucleus of the cell. These are the places where transcription is not active.

Heterochromatin is divided into constitutive and facultative. Constitutive chromatin contains permanently inactive stretches of DNA (large heterochromatin blocks on chromosomes 1, 9, 16 and Y are typical). Facultative heterochromatin contains currently inactive stretches (under certain conditions, they can resume their function). Examples are evolution genes.