Cyclic AMP Response Element Binding protein (CREB) is a basic / leucine zipper transcription factor that binds the cyclic AMP response element (CRE) and activates transcription in response to a variety of extracellular signals including neurotransmitters, hormones, membrane depolarization, and growth and neurotrophic factors. Activation of CREB is dependent upon the phosphorylation of serine 133. Phosphorylation occurs via p44 / 42 MAP kinase and p90RSK and also via p38 MAP kinase and MSK 1. Although CREB will bind DNA independent of its phosphorylation state, only the phosphorylated form is competent as a transcription factor. CREB binding protein (CBP), a transcriptional coactivator that directly interacts with CREB, binds to CREB in the region of serine 133.