Evolution of gastric carcinoma cell-specific aptamer cy-apt 20 by live cell-SELEX. Human gastric carcinoma AGS cells were used as target cell for positive selections and human normal gastric epithelial GES-1 cells as negative cell for counter selections. The selection procedure was monitored by electrophoresis and flow cytometry analyses. Selected DNA aptamers were labled with FITC and their binding affinity to AGS cells were analyzed using flow cytometry. Lib to AGS cells and each identified ssDNA sequence to GES-1, HepG2 and SW620 cells were used as controls. The concentration of these FITC-labeled ssDNA used was 400 nM, and results were presented as mean ± standard error. (A) Ten ssDNA sequences with high binding ability to AGS were identified from the final pool. (B) Four of these ssDNA sequences were found have binding rates more than 60% to AGS cells, but only aptamer cy-apt 20 (arrow notified) have less than 30% binding affinity to non-gastric carcinoma HepG2 and SW620 cells. FITC: fluorescein isothiocyanate. Lib: FITC-labeled unselected library ssDNA. Apt or cy-apt: named identified ssDNA sequence. HepG2: human hepatocellular carcinoma cell. SW620: human colon carcinoma cell.