Fluorescence signal from different molecular beacon designs. In A-D, left and right panels display respectively the undifferentiated (UD) and RA treated (differentiated) cells; top and bottom panels display respectively the epifluorescence images of beacons and the bright field images of the same cells. The same exposure time was used for all beacon fluorescence imaging. A and B. Random beacon used as negative control (A) and MB5 (B) gave very low signal. Signals from MB1, MB6-MB13 were similar to that of MB5. C and D. MB3 (C) and MB4 (D) gave high signal levels, indicating good target accessibility.