miniSOG is a small (106 amino acid residue) genetically encoded photosensitizer that efficiently generates singlet oxygen when illuminated by blue light; it has an absorption peak of 448nm with a shoulder at 473nm, producing green fluorescence with two emission peaks (500 and 528nm). It was engineered by mutagenesis of the LOV2 domain of the Arabidopsis thaliana PHOT2 gene (TAIR:AT5G58140) and does not require any exogenous cofactors (its flavin mononucleotide cofactor is produced endogenously at sufficient levels within cells) (PMID:21483721). The properties of miniSOG can be utilized in a number of different ways (reviewed in PMID:27435584). In fixed tissues, miniSOG can be used as a genetically encoded tag for electron microscopy (EM), to study the ultrastructural location of a tagged protein of interest, or to study the ultrastructure of an organelle or other subcellular structure. Upon illumination by blue light, the singlet oxygen generated by miniSOG locally polymerizes diaminobenzidine (DAB), which results in EM contrast after treatment with osmium (PMID:21483721). miniSOG can also be used to selectively inactivate a protein of interest in living cells, using the chromophore-assisted light inactivation (CALI) technique; in this case, miniSOG is fused to a protein of interest, and upon blue light illumination, the singlet oxygen produced can disrupt the function of this protein by oxidizing susceptible amino acid residues (PMID:23889931). miniSOG has also been used as a light-regulated cell ablation tool (PMID:22532663).