Conjugates of antibodies with photosensitizers are successfully used for the targeted killing of cancer cells bearing particular surface markers by the method known as photoimmunotherapy. However, the chemical conjugation of photosensitizers with antibodies poses a number of problems. Among these are a low reproducibility, aggregation, and the presence of impurities of the unconjugated photosensitizer. Here we describe a method of designing a fully genetically encoded immunophotosensitizer, which consists of the anti-HER2/neu miniantibody 4D5scFv as a targeting molecule and the phototoxic fluorescent protein KillerRed as a photosensitizing molecule. Both domains in the recombinant protein retained their functional properties: a high affinity for the HER2/neu antigen and phototoxicity. The recombinant protein 4D5scFv-KillerRed showed a high specificity for HER2/neu-overexpressing cancer cells and effectively lowered their viability upon irradiation. © 2011 Pleiades Publishing, Ltd.