Several methods exist for creating large libraries of sequence variants. Among the most widely used are subjecting DNA to UV radiation or chemical mutagens, error-prone PCR, degenerate codons, or recombination. Once a large library of variants is created, selection or screening techniques are used to find mutants with a desired attribute. Common selection/screening techniques include FACS, mRNA display, phage display, and "in vitro" compartmentalization. Once useful variants are found, their DNA sequence is amplified and subjected to further rounds of diversification and selection.