Fig. 3. Dividing cells bind only to Shiga toxin (Cy3-ST). Cells were partially synchronized by a double-thymidine block and analyzed for toxin binding 6 hours later. The relative number of small round-shaped cells binding preferentially to Cy3-ST (upper left panel) is significantly higher than in non-synchronized cultures. Note that the dividing cells (arrows) bind only to ST. The flat, extended, irregularly shaped cells bind only to Cy2-CTX (upper right panel). Lower left panel: overlay of the two upper panels. Lower right panel: digital interference contrast.