An enhancer is a DNA sequence that binds proteins (akin to transcription factors) that increase the rate of transcription of a gene. Enhancers may be located either upstream or downstream of the gene they regulate, and they do not have to be in close proximity of it (in terms of the nucleotide sequence). Because of the supercoiled state of DNA in a cell, enhancers may act because they are geometrically close to the gene of interest even if they are hundreds of thousands of bases away from it in terms of nucleotide sequence. Geometric closeness allows the enhancer sequence to interact with general transcription factors and RNA polymerase II at the transcription initiation site of the regulated gene.

Enhancers are bound by activator proteins, which themselves interact with a 'mediator complex' of co-activators. The mediator complex can further stabilise a pre-existing preinitiation complex at the TATA box site of the gene concerned, thereby recruiting RNA polymerase to begin transcription. Thus, there is no direct interaction between the enhancer and the promoter sequence of the gene being regulated.

Because some promoters are only weakly able to interact with transcriptional proteins (i.e. they are rarely transcribed because of this), enhancers might be seen as their 'saviour', ensuring that gene products are still transcribed and translated at suitable rates despite their weak affinity for the transcriptional proteins.