Schematic diagram. The schematic diagram shows that the reaction components were printed on parchment paper, and the fluorescence energy transfer between Eu-cAMP and ULight-anti-cAMP was determined under four different conditions, including: (A) absence of free cAMP; (B) presence of free cAMP; (C) presence of cAMP and PDE4B enzyme; and (D) presence of cAMP, PDE4B enzyme, and PDE4B inhibitor.