In this study, using the bead-based and flow cytometry monitored SELEX technology, we aimed to obtain specific aptamers to AGR2 and study theirs structure and potential function. Beads-based SELEX allowed the use of simple, yet effective, flow cytometry analysis to monitor the progress of the selection, avoiding the tedious, time consuming and radioactive EMSA process [39] [40] [41] [42] [43] . After 14 rounds of selection, we have identified a group of DNA aptamers that specifically bound to AGR2 with high affinities. Structural studies on one of the aptamer sequences, C14B, revealed an intramolecular parallel Gquadruplex, and its structure and binding affinity to AGR2 depend on K + ion intensively. Furthermore, we designed an allosteric molecule beacon AGR2-aMB based on the identified aptamer, which enables simple, sensitive and selective detection AGR2. The aptamer sequences and AGR2-aMB reported in this study are potentially useful tools for early diagnosis and prognosis of cancer and for fundamental research to elucidate the biochemical functions of AGR2.