sentences:  For a randomly generated region of length "n", the number of possible sequences in the library is 4n ("n" positions with four possibilities (A,T,C,G) at each position).The sequences in the library are exposed to the target ligand - which may be a protein or a small organic compound - and those that do not bind the target are removed, usually by affinity chromatography or target capture on paramagnetic beads.The bound sequences are eluted and amplified by PCR to prepare for subsequent rounds of selection in which the stringency of the elution conditions can be increased to identify the tightest-binding sequences.