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CREB is an activator that recognizes the consensus CRE site. It is also found at additional DNA sequences (AP1 and GRE sites) in a complex containing other activators. The transcriptional activity of CREB is dynamically regulated by phosphorylation. The most well studied phosphorylation site is serine 133. Other phosphorylation events can repress its activity. CREB can be bound regardless of its phosphorylation state, however, for some genes CREB is only recruited to their CRE sites upon phosphorylation. CREB binds as a homodimer or heterodimer. Heterodimerization confers repressive activity of CREB as part of a larger complex of repressor proteins.