Fig. 3. Experimental FRET between donor G_M1 and acceptor Gb₃ molecules. E increased as the concentration of acceptor Cy5-Stx1B used to label the cell was increased from D:A 1:1 (b, closed circles) to D:A 1:3 (a, open circles). In these experiments E increased from 15% to 33%, as D:A was increased. FRET does not occur between G_M1 and Gb₃ molecules after MßCD treatment (c, open triangles).