ASAP1 is a genetically encoded fluorescent voltage indicator; fluorescence is responsive to membrane potential, with depolarization resulting in decreased fluorescence. ASAP1 can reliably detect single action potentials and sub-threshold potential changes, and can track trains of action potentials up to 200Hz. ASAP1 consists of a voltage-sensing domain (from a Gallus gallus voltage sensitive phosphatase gene) with the circularly permuted 'OPT' variant of sfGFP inserted into the extracellular S3-S4 loop in between residues 147 and 148. In addition, the voltage-sensing domain contains a R153Q amino acid substitution (PMID:24755780). For detailed kinetic and spectral properties, see PMID:24755780.