A method of introducing diversity into a reference protein sequence, the method comprising: (a) selecting a plurality of mutant proteins having a beneficial property, wherein the mutant proteins are derived from the reference protein sequence, and wherein the selection comprises: (i) selecting a plurality of mutant proteins having the beneficial property from a first sub-library having mutations only in a first region of the reference protein sequence to provide a first selected group of mutants; and (ii) selecting a plurality of mutant proteins having the beneficial property from a second sub-library having mutations only in a second region of the reference protein sequence to provide a second selected group of mutants; (b) isolating a first group of nucleic acid segments, each nucleic acid segment in the first group encoding the first region of the first selected group of mutants but not encoding substantially any other region of the reference protein sequence, and each nucleic acid segment in the first group encoding its own distinct mutation; (c) isolating a second group of nucleic acid segments, each nucleic acid segment in the second group encoding the second region of the second selected group of mutants but not encoding substantially any of the first region of the reference protein sequence, and each nucleic acid segment in the second group encoding its own distinct mutation; and (d) assembling at least the isolated nucleic acid segments from (b) and (c) into full length nucleic acid sequences encoding new mutant proteins, wherein (b) is performed without determining sequences of members of the first group of nucleic acid segments. (a) selecting a plurality of mutant proteins having a beneficial property, wherein the mutant proteins are derived from the reference protein sequence, and wherein the selection comprises: (i) selecting a plurality of mutant proteins having the beneficial property from a first sub-library having mutations only in a first region of the reference protein sequence to provide a first selected group of mutants; and (ii) selecting a plurality of mutant proteins having the beneficial property from a second sub-library having mutations only in a second region of the reference protein sequence to provide a second selected group of mutants; (i) selecting a plurality of mutant proteins having the beneficial property from a first sub-library having mutations only in a first region of the reference protein sequence to provide a first selected group of mutants; and (ii) selecting a plurality of mutant proteins having the beneficial property from a second sub-library having mutations only in a second region of the reference protein sequence to provide a second selected group of mutants; (b) isolating a first group of nucleic acid segments, each nucleic acid segment in the first group encoding the first region of the first selected group of mutants but not encoding substantially any other region of the reference protein sequence, and each nucleic acid segment in the first group encoding its own distinct mutation; (c) isolating a second group of nucleic acid segments, each nucleic acid segment in the second group encoding the second region of the second selected group of mutants but not encoding substantially any of the first region of the reference protein sequence, and each nucleic acid segment in the second group encoding its own distinct mutation; and (d) assembling at least the isolated nucleic acid segments from (b) and (c) into full length nucleic acid sequences encoding new mutant proteins,