Epac-S-H189 is a genetically encoded fluorescent cAMP sensor. It is a fusion protein that consists of a FRET (fluorescence resonance energy transfer) donor (mTurquoise2 with a truncation of the C-terminal 11 amino acids) and a 'dark' (non-emitting) FRET acceptor (two tandem copies of a circularly permuted version of Venus which each contain the Y145W mutation to reduce fluorescence emission) which are separated by sequence from the Homo sapiens RAPGEF3 (Epac1) cAMP-binding protein. The RAPGEF3 sequence lacks the N-terminal DEP domain, is catalytically dead (contains amino acid replacements T781A and F782A) and contains a Q270E mutation which increases the affinity to cAMP. The RAPGEF3 portion undergoes a conformational change upon binding to cAMP, which causes a robust decrease in FRET. The use of the dark FRET acceptor in this sensor means that it is optimized for FLIM (fluorescent lifetime imaging detection (PMID:25875503).