RNAi, heterochromatin and the cell cycle.

For many decades after its initial characterization, heterochromatin was considered to be transcriptionally inert, but newer work indicates that this highly condensed chromosomal material is transcribed, and rapidly silenced, by an orchestrated sequence of events directed by RNA interference (RNAi). Recent studies shed light on the timely assembly and inheritance of heterochromatin within a short period during the cell cycle, thereby providing an explanation for how 'silent' heterochromatin can be transcribed during the S phase of the cell cycle. Together, these findings suggest a model of RNAi-directed epigenetic inheritance.