Strep-Tactin® conjugated to allophycocyanin (APC) binds to Strep-tag®II- or Twin-Strep-tag®- fusion proteins and is optimized for flow cytometry of cells. Strep-Tactin® is a streptavidin variant and therefore it is a tetrameric molecule that also has an affinity to biotin. Cells can either be stained directly if they express a strep-tagged surface protein or indirectly via a strep-tagged protein that subsequently binds to the cells (e.g. for sorting antigen-specific B cells). Independent from the chosen method, biotin addition will cause the release of Strep-Tactin® APC from the cell surface. This enables the use of selected cells for further downstream applications.