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Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the "switching off" of a gene by a mechanism other than genetic modification. That is, a gene which would be expressed ("turned on") under normal circumstances is switched off by machinery in the cell. Gene silencing occurs when RNA is unable to make a protein during translation.
Transcriptional gene silencing is the result of histone modifications, creating an environment of heterochromatin around a gene that makes it inaccessible to transcriptional machinery (RNA polymerase, transcription factors, etc.).
Post-transcriptional gene silencing is the result of mRNA of a particular gene being destroyed or blocked. The destruction of the mRNA prevents translation to form an active gene product (in most cases, a protein). The blocking of the gene occurs through the activity of silencers, which bind to repressor regions. A common mechanism of post-transcriptional gene silencing is RNAi.
Both transcriptional and post-transcriptional gene silencing are used to regulate endogenous genes. Mechanisms of gene silencing also protect the organism's genome from transposons and viruses. Gene silencing thus may be part of an ancient immune system protecting from such infectious DNA elements.
Specific studies of gene silencing 
There are several more terms related to specific topics of gene silencing:
Transcriptional Gene Silencing:
- Genomic Imprinting
- Transposon silencing
- Transgene silencing
- Transcriptional gene silencing
- position effect
- RNA-directed DNA methylation
Post-transcriptional Gene Silencing:
Meiotic gene silencing:
Cellular components of gene silencing:
- RNAiAtlas - database of siRNA libraries and their target analysis results
- Science project: Transgenic apple varieties Approaches to preventing outcrossing – possible effects on micro-organisms
- Gene silencing at the US National Library of Medicine Medical Subject Headings (MeSH)
- Research project: New Cost-effective method for gene silencing
- van Leeuwen F, Gottschling DE. (2002). "Assays for gene silencing in yeast.". Methods Enzymol. 350: 165–86. doi:10.1016/S0076-6879(02)50962-9. PMID 12073311.