Promoters reside at the beginning of the gene and serve as the site where the transcription machinery assembles and transcription of the gene begins.  Enhancers turn on the promoters at specific locations, times, and levels and can be simply defined as the “promoters of the promoter.” Silencers are thought to turn off gene expression at specific time points and locations.  Insulators, also called boundary elements, are DNA sequences that create cis-regulatory boundaries that prevent the regulatory elements of one gene from affecting neighboring genes.  The general dogma is that these regulatory elements get activated by the binding of   transcription factors, proteins that bind to specific DNA sequences, and control mRNA transcription. There could be several transcription factors that need to bind to one regulatory element in order to activate it. In addition, several other proteins, called transcription cofactors, bind to the transcription factors themselves to control transcription.