Existing models for the function of enhancers. The four existing models describing gene regulation by enhancers are depicted. (A) The tracking model, where a transcription factor (purple hexagon) loads onto the enhancer and tracks along the chromatin fibre towards the promoter, where it stimulates transcription by association with the polymerase (pink oval). (B) The linking model, where the loaded transcription factor drives polymerization of proteins in the direction of the promoter. (C) The relocation model, where a gene relocates to nuclear subcompartments (pink halo) favouring enhancer-promoter interactions, and so transcription. (D) The looping model, where the enhancer comes into proximity with the relevant promoter due to protein-protein interactions. This loops out the intervening chromatin and triggers transcriptional activation.