The procedures of selection were as follows. The ssDNA pool (200 pmol) dissolved in binding buffer (200 mL, 16PBS buffer containing 137 mM NaCl, 2.7 mM KCl, 10 mM Na 2 HPO 4 and 2.0 mM KH 2 PO 4 , pH 7.4) was denatured by heating at 95uC for 5 min, then quickly cooled on ice for 10 min, and subsequently incubated for another 10 min at room temperature before binding. The ssDNA pool was then incubated with negative GST beads (1.0610 5 beads) for counter selection to remove sequences non-specifically binding to GST beads. After filtering with a homemade filter column, the filtrate was incubated with positive AGR2-GST-beads (1.0610 5 beads) at 37uC for 45 min. The unbound or nonspecifically bound oligoes were removed by filtration. The sequences bound to the target-coated beads were then amplified by PCR with FAM and biotin-labeled primers (5-15 cycles of 0.5 min at 94uC, 0.5 min at 53uC, and 0.5 min at 72uC, followed by 5 min at 72uC; the Easytaq plus polymerase and dNTPs were obtained from Transgen Beijing). After denaturation in NaOH (0.1 M), the selected sense ssDNA was separated from the biotinylated antisense ssDNA strand on streptavidin-coated sepharose beads (GE healthcare) and used for next round selection. For the first-round selection, the amount of initial ssDNA pool was 5 nmol, dissolved in 500 mL binding buffer, and the counter-selection step was eliminated. To acquire aptamers with high affinity and specificity, the selection strength was enhanced gradually by increasing the number of washes (from three to ten times with 200 mL 16PBS buffer each) and decreasing the amount of the ssDNA library per round (from 200 to 150 pmol).