In a typical round of biopanning, ~2 x 1011 input phage are reacted with the target, and between 103 and 107 total phage are eluted off following washing. This corresponds to an enrichment of 104 to 108-fold per round. Since the library contains ~2 x 109 different clones, the eluted pool of phage should in theory be fully enriched in favor of binding sequences after only 2 or 3 rounds. Once this point is reached, further rounds of amplification and panning will result only in selection of phage that have a growth advantage over the library phage. For example, vanishingly small levels of contaminating environmental wild-type phage (less than one part per billion) will completely overtake the pool if too many rounds of amplification are carried out, regardless of the strength of the in vitro selection.