Since C14B is the best aptamer we obtained from the four sequences tested, it was choses for future optimization and characterization. The length of C14B is 87mer, which is disadvantageous for future applications because it is inconvenient and expensive to synthesize such a long sequence. To identify the binding region of the aptamer, the marginal sequences of C14Bwas gradually truncated. Two truncated sequencesC14B0 and C14B1 from C14B were shown in Table 1 . Subsequent binding affinity experiments revealed that both C14B0 (8.563.6 nM) and C14B1 (19.165.1 nM) have similar K d to AGR2 as that of original C14B (13.167.2 nM). The majority of eliminated sequences were primer sequences, implying that the binding region of the aptamer was the middle of random sequences and the primer sequences do not or contribute little to the binding affinity of aptamer. C14B1 has five portions of poly-G and we designed five truncated sequences by removing one of poly-G portion each (Table S2) . Removal of any poly-G portion would destroy its binding affinity to certain extend ( Figure S3) , suggesting the whole G-motif is required for aptamer binding. Taking together, these results indicate that C14B1, which was only 33 mer, was the essential binding region to AGR2. Thus, C14B1 was applied for further characterization and probe design.