iLACCO1 is a genetically-encoded sensor for lactate. It contains the L-lactate binding domain (LBD) of the Escherichia coli lldR gene (without the DNA-binding domain), into which a circularly permuted green fluorescent protein (cpGFP) has been inserted at position 187. A 3 residue linker joins the C-terminal end of the cpGFP to the lldR-LBD sequence, and a number of amino acid substitutions have been introduced into the cpGFP and lldR-LBD coding sequences by directed evolution (FBrf0258931).