Heterochromatin is a tightly packed form of DNA. Its major characteristic is that transcription is limited. As such, it's a means to control gene expression, through regulation of the transcription initiation. Structure Chromatin is found in two varieties: euchromatin and heterochromatin. Originally, the two forms were distinguished cytologically by how darkly they stained - the former is lighter, while the latter stains darkly, indicating tighter packing. Heterochromatin is usually localized to… (More on Heterochromatin)