Abstract
How does the cell maintain the huge areas of the genome containing DNA repeats in its heterochromatin form? As [Thomas Jenuwein][1] discusses in his indepth Perspective, new clues have arrived from the fission yeast ([Hall][2] et al .) and the ciliated protozoan Tetrahymena . These organisms reveal that small heterogeneous RNAs formed at DNA repeats in the nucleus are important for initiating and maintaining heterochromatin. [1]: http://www.sciencemag.org/cgi/content/full/297/5590/2215 [2]: http://www.sciencemag.org/cgi/content/short/297/5590/2232