Silencer is the DNA element that, upon binding with transcription factors (repressors), can repress transcription. In prokaryotes, silencers are known as operators, found in many genes such as lac operon and trp operon. In eukaryotes, the following genes have been demonstrated to contain silencers:
In a few cases, a DNA element may act either as an enhancer or a silencer, depending on the binding protein. For example, certain genes contain an element called E box (consensus CACGTG) which can bind either Max/Myc dimer or Max/Mad dimer. The Max/Myc dimer activates transcription, whereas the Max/Mad dimer suppresses transcription of these genes.