After 14 rounds of selection, the enriched DNA pool was cloned and sequenced. The sequencing data for clones were analyzed by using sequencing analysis software Clustal W 6.0 [44] . The sequences were grouped based on the homology similarity of the DNA sequences from individual clone. Among the sequences from 62 clones, there were two subfamilies each containing multiple sequences with common features. One subfamily is guanosine-rich sequences (22 clones) , and the other is thymine-rich sequences (40 clones) ( Figure S1 ). Four sequences were chosen and synthesized for further characterization (Table S1 ): three sequences from Grich subfamily, C14A (appeared 2 times), C14B (appeared 3 times) and C14C (appeared 5 times), and one sequence C14D (appeared 2 times) from T-rich subfamily. Only one T-rich sequence was chosen because of T-rich sequences tend to from non-rigid tertiary structures and the chance of being aptamer was thought to be very low. As shown in Figure 2c , C14B can bind AGR2 with the highest affinity (K d = 13.167.2 nM). Titration of C14B to GSTbeads revealed no observable binding, establishing that the binding target of C14B was indeed AGR2. Other three sequences have similar binding constants to AGR2 but a little weaker than C14B, in which the K d of C14A is 20.965.2 nM, C14C is 44.667.0 nM and C14D is 48.4615.6 nM. ( Figure S2 ).