B/B-like fragment targeting for the purposes of photodynamic therapy and medical imaging

The invention provides compositions and methods for use in delivering a substance of interest to a targeted cell. The substance of interest is associated with a targeting fragment of a toxin molecule or a lectin that specifically binds to a cell surface receptor, such as the B subunit of an A/B type toxin molecule. The substance of interest may be a photosensitizing agent, in which case the cell (e.g. a cancer cell) may be killed by exposure to light after delivery of the agent. Alternatively, the substance of interest may be a visualizing agent that enhances visualization of the targeted cell.