Heterochromatin is a tightly packed form of DNA, which comes in multiple varieties. Because it is tightly packed, it is inaccessible to polymerases and is therefore not transcribed, (or so we thought. According to Volpe et al, and many other papers since, we see that much of this DNA is in fact transcribed, however it is continuously turned over via a RITS pathway) . These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin. Both play a role in the expression of genes.