Schematic summarizing the roles of different classes of regulatory elements. The proximal promoter (dark blue) is located in the immediate vicinity (-250 bp to +250 bp) of the gene's transcription start site (TSS; indicated by the arrow pointing to the right). Additional elements up to 5 kb upstream of this region are still considered part of the promoter (light blue). Promoters are enriched for transcription factor binding sites (TFBS) and are thought to serve as tethering elements for enhancers. The formation of an enhancer-promoter loop activates transcription of the target gene. MicroRNAs (miRNAs) can modulate the abundance of specific mRNA transcripts by binding to their 3' untranslated regions (UTRs). Silencers are thought to have the opposite effect to enhancers, turning off the expression of genes in specific tissues and at specific time points. Insulators are thought to act as barriers, preventing enhancers and silencers from regulating neighboring genes.