An uncertain silence.

Transcriptional silencing is the most well-studied epigenetic phenomenon in yeast. In Saccharomyces cerevisiae, silencing has recently been found at previously unidentified loci. In addition to the silent mating-type loci and telomeres, genes within the ribosomal DNA and, perhaps, at undefined aging loci are silenced. Efficiency of silencing at different loci varies and is affected by competition between the loci and by the involvement of different factors in distinct protein complexes. The recent discovery of conserved gene families encoding proteins related to modulators of acetylation and deacetylation suggests mechanisms for differential regulation of silencing at known loci and the existence of additional, as yet undiscovered, silenced loci.