Binding Affinity and Structure Stability of C14B1 is K + Dependent Many DNA aptamers have been found to form G-quadruplex structures [49] [50] [51] . It is well established that the existence of a monovalent cation (especially potassium) in the center of these tetrads can significantly stabilize G-quadruplexes [52] [53] [54] . Thus, we investigated how a cation would affect the structural stability of C14B1 and its binding activity towards to AGR2. As shown in Figure 4a , the stability of the C14B1 G-quadruplex structure is strongly dependent on the presence of the monovalent ion. With 60 mM of KCl, strong CD peaks were observed, suggesting the formation of stable G-quadruplex structure. Replacing KCl with the same concentration of LiCl, NaCl, and MgCl 2 led to a dramatic intensity decrease in CD.