An extracellular protein that has four high affinity binding sites for biotin. Streptavidin is widely utilized in fluorescence microscopy applications due to the ease with which a conjugate fluorescent moiety can be coupled to the protein. Check for active clinical trialsusing this agent. OR A small bacterial protein that binds with high affinity to the vitamin biotin. This streptavidin-biotin combination can be used to link molecules such as radioisotopes and monoclonal antibodies together. These bound products have the property of being attracted to, and attaching to, cancer cells, rather than normal cells. The radiolabeled products are more easily removed from the body, thus decreasing their toxicity.