A method for the synthesis of a desired circular nucleic acid molecule, said method comprising: (a) providing a plurality of cassettes, each cassette comprising a nucleotide sequence of a portion of the desired nucleic acid molecule, wherein: (i) each cassette is 2 to 10kb in length; (ii) members of the plurality of the cassettes contain overlapping portions of nucleotide sequence; and (iii) the cassettes, when combined according to the overlapping portions, provide subsets of the complete nucleotide sequence of the desired nucleic acid molecule; (b) combining said plurality of cassettes and optionally an additional vector DNA in vitro in one or more steps to obtain a plurality of resulting subsets wherein: (i) said subsets contain overlapping portions of nucleotide sequence of 80bp or up to 360bp; and (ii) the subsets, when assembled according to the overlapping portions, provide the complete nucleotide sequence of the desired circular nucleic acid molecule; and wherein the subsets or the additional vector DNA encode an origin of replication and/or a yeast centromere comprising overlapping portions of the nucleotide sequence; and (c) transfecting S. cerevisiae yeast host cells with a mixture of 10 or more fragments of DNA comprising subsets of from 15kb to 200kb or 50kb to 500kb in size, wherein the 10 or more fragments and optionally an additional vector DNA are assembled in vivo in the yeast host cell to obtain the complete desired circular nucleic acid molecule, wherein said desired circular nucleic acid molecule further includes an origin of replication operable in the host cell, wherein said origin of replication was present in the cassette, fragment or the vector DNA. (a) providing a plurality of cassettes, each cassette comprising a nucleotide sequence of a portion of the desired nucleic acid molecule, wherein: (i) each cassette is 2 to 10kb in length; (ii) members of the plurality of the cassettes contain overlapping portions of nucleotide sequence; and (iii) the cassettes, when combined according to the overlapping portions, provide subsets of the complete nucleotide sequence of the desired nucleic acid molecule; (i) each cassette is 2 to 10kb in length; (ii) members of the plurality of the cassettes contain overlapping portions of nucleotide sequence; and (iii) the cassettes, when combined according to the overlapping portions, provide subsets of the complete nucleotide sequence of the desired nucleic acid molecule; (b) combining said plurality of cassettes and optionally an additional vector DNA in vitro in one or more steps to obtain a plurality of resulting subsets wherein: (i) said subsets contain overlapping portions of nucleotide sequence of 80bp or up to 360bp; and (ii) the subsets, when assembled according to the overlapping portions, provide the complete nucleotide sequence of the desired circular nucleic acid molecule; and wherein the subsets or the additional vector DNA encode an origin of replication and/or a yeast centromere comprising overlapping portions of the nucleotide sequence; and (i) said subsets contain overlapping portions of nucleotide sequence of 80bp or up to 360bp; and (ii) the subsets, when assembled according to the overlapping portions, provide the complete nucleotide sequence of the desired circular nucleic acid molecule; and wherein the subsets or the additional vector DNA encode an origin of replication and/or a yeast centromere comprising overlapping portions of the nucleotide sequence; and (c) transfecting S. cerevisiae yeast host cells with a mixture of 10 or more fragments of DNA comprising subsets of from 15kb to 200kb or 50kb to 500kb in size, wherein the 10 or more fragments and optionally an additional vector DNA are assembled in vivo in the yeast host cell to obtain the complete desired circular nucleic acid molecule, wherein said desired circular nucleic acid molecule further includes an origin of replication operable in the host cell, wherein said origin of replication was present in the cassette, fragment or the vector DNA.