Cell-SELEX. (1) Target cells are incubated with a complex library consisting of up to 1015 individual single stranded DNA molecules (ssDNA). (2) The separation of cell-binding nucleic acids from nonbinding species can be performed by different methods: FACS (2a), centrifugation (2b) or washing (rinsing) steps (2c), respectively. (3) Cell-binding DNA molecules—aptamers—are eluted and (4) subsequently amplified by PCR using two specific primers. The 5′-end of the reverse primer carries a biotin. Additionally, a fluorescently labeled forward primer is used for PCR, if FACS is used for separation. After strand displacement the enriched DNA library is used for further selection rounds. This cell-SELEX procedure ends up with aptamers that bind specifically to the surface of target cells.
