Gene Silencer
 
A silencer is a cis-acting regulatory sequence that lowers the rate of initiation of transcription of eukaryotic genes. Silencer elements have been identified from a variety of organisms. The first and best-characterized silencer was identified near the promoters for mating-type genes at the HML and HMR loci in the yeast Saccharomyces cerevisiae (1). The yeast-mating-type genes, a and a, at HML and HMR are transcriptionally silent and become active only when they replace the mating-type allele present at the MAT locus during the process known as mating-type switching. The silenced mating-type genes require E regulatory sites known as HMLE and HMRE, respectively. Molecular analyses have identified a DNA element at the E regulatory site, called the silencer, capable of repressing transcription of a target promoter in cis. This element comprises binding sites for regulatory proteins. Interactions of the silencer element with these proteins are necessary and sufficient for forming the repressive chromatin structure (2, 3). This element was named a silencer because of its similarity to transcriptional enhancers, despite their opposite effects on transcription: (i) it functions in either orientation; (ii) it exerts a silencing effect relatively independently of its position with respect to its target promoter; (iii) it is capable of repressing promoters other than its normal target. Subsequently, however, the term silencer has been also used to describe negatively acting regulatory elements that do not conform to the criteria used in the original definition (4).
References
1. A. H. Brand et al. (1985) Cell 41, 41–48. 
2. S. Loo and J. Rine (1995) Annu. Rev. Cell Dev. Biol. 11, 519–548. 
3. S. G. Holmes, M. Braunstein, and J. R. Broach (1996) in V. E. A. Russo, R. A. Martienssen, and A. D. Riggs, eds. Epigenetic Mechanisms of Gene Regulation. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, pp. 467–487. 
4. S. Ogbourne and T. M. Antalis (1998) Biochem. J. 331, 1–14.