Photosensitizers are chromophores that generate reactive oxygen species (ROS) upon light irradiation. They can be used for precise inactivation of selected proteins in chromophore-assisted light inactivation (CALI) technique and for the light-induced cell killing, for example in photodynamic therapy. Besides KillerRed protein, all known to date photosensitizers are chemical compounds that must be introduced into living systems exogenously. Red fluorescent protein KillerRed is the first genetically-encoded photosensitizer [Bulina et al., 2006a]. Unlike chemical analogs, KillerRed can be directly expressed by target cells, both individually and in fusion with a target protein. It shows no cell toxic effects before light activation. Upon green-light irradiation, KillerRed generates ROS that damage the neighboring molecules.