Fig. 4. Cells in S-phase exhibt low binding of both toxins. Cells were pretreated with BrdU, exposed to Cy2-CTX and Cy3-ST and analyzed by fluorescence microscopy as in Fig. 1a. Cells were then fixed, permeabilized and analyzed for BrdU incorporation using anti-BrdU-antibodies. Cells strongly stained for BrdU (S-phase) show little or no binding of CTX (CTX) or ST (ST).