Nucleic Acid Assembly System

The present invention relates to a method for the preparation of a library of host cells, a plurality of which comprise an assembled polynucleotide at a target locus, which method comprises: (a) providing a plurality of polynucleotides comprising two or more polynucleotide subgroups, wherein: (i) a plurality of polynucleotides in each polynucleotide subgroup comprises sequence encoding a peptide or polypeptide and/or a regulatory sequence; (ii) a plurality of peptides or polypeptides encoded by, or a plurality of regulatory sequences comprised within, each polynucleotide subgroup share an activity and/or function; (iii) at least one polynucleotide subgroup comprises at least two non-identical polynucleotide species; (iv) a plurality of polynucleotides of each polynucleotide subgroup comprises sequence enabling homologous recombination with a plurality of polynucleotides from one or more other polynucleotide subgroups; and (v) a plurality of polynucleotides in two polynucleotide subgroups comprise a nucleotide sequence enabling homologous recombination with a target locus in host cells; and (b) assembling the plurality of polynucleotides at the target locus by homologous recombination in vivo in host cells, thereby to generate a library of host cells, a plurality of which comprise an assembled polynucleotide at the target locus. The assembled polynucleotides may be recovered, thereby to prepare a library of nucleic acids.