This is a description of heterochromatin on wikipedia. I am amazed that it is basically correct and even covers most of the concepts. It is a good introduction of this interesting kind of chromatin.
However, I have to correct some of the obvious mistakes.
1. first paragraph. "Its major characteristic is that transcription is limited." This is definitely going to change with the current research showing transcription from heterochromatin in many species. At least it is not the "major" characteristic anymore. The concept of heterochromatin is being reshaped dramatically recently.
2. still first paragraph. "As such, it is a means to control gene expression, through regulation of the transcription initiation." this is correct but certainly not the most important function of heterochromatin. Also it is not clear if the regulation is only through transcription initiation.
3. structure: about genes in heterochromatin regions, there are some, especially in fruit fly. About its replication timing, in most species heterochromatin replicates late in S phase. But in fission yeast, it replicates very early. --Joel Huberman nicely demonstrated this years ago.
4. On the potential involvement of RNAi in heterochromatin formation in higher eukaryotes like mammalian systems, the evidence is very controversy currently. Almost all supporting evidence came from one single lab. My opinion is that it is not as general. There might be some similarity, but models of fission yeast cannot be simply adopted to mammalian systems.
I will expand this hot area later on.
5. One important thing missing is the molecular hallmark of heterochromatin, i.e. the histone H3 methylation at lysine 9 (H3K9)- HP1 (heterochromatin protein 1) interaction and the histone H3K9 methytransferases.
Ok. Enough for wikipedia.