We hypothesize that the dependence on probe concentration is because the binding of YOYO-1 is a twostep process. The first step is being adsorbed onto the DNA and the second step is intercalating into the base pairs. At a low concentration limit, all YOYO-1 adsorbed onto the DNA at random orientations and locations have enough time to re-organize themselves and eventually intercalate into the base pairs and light up under the microscope. At high concentrations, however, the next molecule will hit and occupy the intercalation orientation thus those bound in the wrong places never have a chance to light up. As such, the effective binding area at low probe concentrations is the whole DNA which is reduced to the actual binding area at high probe concentrations. Our measurements with different probe concentrations distinguish the adsorption and absorption cross-sections in this example case.