EP300 (E1A binding protein p300) is an adenovirus E1A-associated cellular p300 transcriptional co-activator protein. It is a histone acetyltransferase that regulates transcription via chromatin remodeling and is important in the processes of cell proliferation and differentiation. It mediates cAMP-gene regulation by binding specifically to phosphorylated CREB protein. p300 has also been identified as a co-activator of HIF1A (hypoxia-inducible factor 1 alpha), and thus plays a role in the stimulation of hypoxia-induced genes such as VEGF. Defects in this protein cause of Rubinstein-Taybi syndrome and may also play a role in epithelial cancer.