Figure 1.

General scheme of in vitro selection procedures. The starting population of variant molecules enters in the selection cycle. Sequence variants are separated into active, those that satisfy selection criteria, and inactive molecules. Active molecules are selectively amplified by PCR resulting in the production of a new pool of molecules that can be the input of a new selection cycle. An additional reverse transcription and in vitro transcription steps are required, before and after PCR, respectively, when the selection is performed on RNA molecules. Alternatively, active and inactive resulting pools can be cloned and analyzed.