Abstract: Methods for preparing DNA libraries for protein engineering are described. The methods use programmable nucleases, such as from Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) systems, to generate diverse protein engineering libraries.

Abstract: Class 2 CRISPR-Cas nucleoprotein complexes are disclosed comprising a Class 2 CRISPR-Cas protein, a CRISPR-Cas associated polynucleotide lacking a spacer element (casPN), and a distinct spacer element sequence polynucleotide (sesPN) comprising a target nucleic acid binding sequence. These complexes are capable of site-directed binding to a target nucleic acid complementary to the target nucleic acid binding sequence of the sesPN. The Class 2 CRISPR-Cas nucleoprotein complexes facilitate site-specific modifications, including cleavage and mutagenesis, of a target nucleic acid sequence. Polynucleotide sequences, expression cassettes, vectors, compositions, and kits for carrying out a variety of methods are also described.