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An enhancer is just a synonym for an upstream activating sequence (UAS). Regarding transcription and in general genetics an enhancer is a 50-150 base pair area of DNA that can be bound by activators to initiate transcription. Location is usually irrelevant for enhancers; they can occur downstream or upstream from the start site and can function irrespective of location (i.e. can be thousands of base pairs away).

There are two classes of enhancers:

Class I: The Common Sequence Elements which bind abundant, constitutively active factors (i.e. the expression of these factors are constantly turned on, and not in response to any particular signal/stimuli). An example is the GC box (sequence GGGCGG) which binds the factor Sp1[1].

Class II: The Response Elements which bind factors that are induced by particular stimuli. For example, the Serum Response Element (sequence TGACTCA) binds the Serum Response Factor (SRF) made in response to growth factor in serum[2].


  1. Grossman S, Engreitz J, Ray J, Nguyen T, Hacohen N, Lander E. Positional specificity of different transcription factor classes within enhancers. Proceedings of the National Academy of Sciences. 2018;115(30):E7222-E7230.
  2. Sullivan A, Benner C, Heinz S, Huang W, Xie L, Miano J et al. Serum Response Factor Utilizes Distinct Promoter- and Enhancer-Based Mechanisms To Regulate Cytoskeletal Gene Expression in Macrophages. Molecular and Cellular Biology. 2010;31(4):861-875.
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