Screening and identification of osteosarcoma cell aptamers
Small oligonucleotide ligands of DNA or RNA have been extensively researched and used in the medical and other bioscience fields [1-5]. Because small oligonucleotide ligands of DNA or RNA can fold into 3D structure and recognize and interact with target, shch as amino acids [6,7], peptides , bacteria , proteins [10-12], and a whole cells [13-18], they show abilities of selective binding and affinity same as antibodies. Molecules of oligonucleotide DNA or RNA can be chemically synthesized and modified to incorporate some functional groups, such as biotin, amino and carboxyl which do not affect the recognition and binding of the aptamers to the target. In addition, oligonucleotide ligands of DNA or RNA have low toxicity, immunogenicity and blood residence time compared with antibodies . Therefore, small oligonucleotide ligands of DNA or RNA have been used for probers or aptamers of tumour marker, such as platele derived growth factor (PDGF), vascular endothelial growth factor (VEGF), human epidermal growth factor receptor 3(HER3). Some aptamers were screened from whole cancer cells, such as acute lymphoblastic leukemia cells, liver cancer cells and small cell lung cancer . Aptamers were usually generated by SELEX. Capillary electrophoresis-SELEX [19-20], complex target-SELEX
[21,22], Subtractive-SELEX  and cell-SELEX have been developed according to the purpose of research and application. Cell-SELEX technology includes the incubation of the ssDNA or RNA library pool with target cells. After incubation, the sequences binding to the target cells were recovered from cells by heating the complex of cells-ssDNA. Sequences which had been recovered were amplified by PCR to generate a new ssDNA or RNA pools for next round of selection. This process was repeated until the sequences that specifically recognize the target cells were enriched.