CREBtide is a synthetic substrate for PKA (Km=3.9 μM), which is based on the phosphorylation sequence in d-CREB (cAMP response element binding protein). The rate of CREBtide phosphorylation was at least as great as that of delta-CREB, indicating that CREBtide retained the determinants of delta-CREB which were responsible for substrate efficacy.