Tag:iLID is an engineered light-regulated specific binding tool based on the LOV2 flavin binding domain (including the carboxy-terminal Jα helix) of the Avena sativa Phototropin 1 (PT1) gene. Seven amino acid residues from the Escherichia coli ssrA gene (ECOGENE:EG30100) have been inserted near the C-terminal end of the Jα helix and Tag:iLID contains a number of other mutations selected to improve its utility. The ssrA residues present in Tag:iLID normally bind the adaptor protein encoded by the Escherichia coli sspB gene (ECOGENE:EG10978). In the dark, binding of the SsrA sequence in Tag:iLID to SspB protein is sterically inhibited, while upon blue light irradiation, unfolding of the Jα helix allows the SsrA peptide sequence to bind to SspB, resulting in a heterodimer. Tag:iLID increases its affinity for SspB by over 50-fold upon light stimulation (PMID:25535392). This property can be used to bring together two tagged proteins of interest in vivo (one tagged with Tag:iLID and the other with Tag:SSPB) in a light-dependent manner, for example to reconstitute a functional protein by bringing together complementary split system components, or to target a protein of interest to a particular subcellular location.