Further examination revealed that C14B1 has multiple stretches of guanines (CGGGTGGGAGTTGTGGGGGGGGGTGG-GAGGGT). It is well-known that guanine-rich sequences can fold into four-stranded secondary structures called quadruplexes. According to G-Quadruplex prediction formula d(G 3+ N 1-7 G 3+ N 1- Figure 1 . Schematics of systematic evolution of DNA aptamers against AGR2. In sepharose-beads-based SELEX process for protein AGR2, the GST-beads were incubated with the ssDNA library for counter-selection to remove nonspecific sequences. The unbound DNAs were then incubated with AGR2-GST-beads for target-selection. After harshly washing, the AGR2-specific DNA sequences were subsequently amplified by PCR for the next round of selection, or for cloning and sequencing to identify individual aptamers after flow cytometry analysis. doi:10.1371/journal.pone.0046393.g001 single stranded G-quadruplex structure, the dependence of melting temperature on the concentration of C14B1 was studied [48] . The melting temperature of C14B1 was found to be 59uC, which was independent of oligonucleotide concentration ( Figure  S4 ), indicating that the aptamer forms an intramolecular Gquadruplex.