An 87-nucleotide (87-nt) single-stranded DNA (ssDNA) library with 45 random bases flanked by two primer sequences (22-nt and 20-nt) was subjected to the SELEX procedure. The library was first allowed to interact with excess negative control beads, and only the DNA sequences that did not bound to the GST-beads were collected. The collected sequences were then incubated with AGR2-GST-beads. After rigorous washing, those sequences that either did not bind, or only weakly bound to the target were discarded. Only the sequences that bound strongly enough were retained on beads, and the bead-ssDNA complexes were collected and amplified by PCR for the next round of selection. After multiple rounds of selection, the subtraction process efficiently reduced the DNA sequences that bound to the GST beads, while those AGR2-specific aptamer candidates were gradually enriched. The progress of the selection process was monitored by flow cytometry. The stronger binding of DNA library to AGR2, the more FAM labeled sequences bound to the beads, thus the higher fluorescence intensity the beads would emit. With the increasing number of selection cycles, steady increases in fluorescence intensity on the target beads were observed (Figure 2a) . The binding affinity of the enriched library after 14 rounds of selection was determined to be in the nanomolar range (K d = 64.165.4 nM), while there was no observable binding of the library to control beads ( Figure 2b ). These results suggested that the DNA aptamers specifically recognizing AGR2 were enriched during the selection process.