Laconic is a genetically-encoded FRET nanosensor for lactate. It contains the full-length lldR gene from Escherichia coli (which contains a lactate binding domain), flanked by the fluorescent proteins mTFP1 and Venus, without artificial linkers. mTFP1 and Venus act as FRET donor and acceptor respectively. Binding of lldR by lactate results in a conformational change which decreases FRET efficiency (PMID:23469056).