A method for producing a collection of nucleic acids, wherein each nucleic acid encodes a human immunoglobulin variable domain comprising a plurality of complementarity determining region 3 sequences isolated separately from the immunoglobulin variable domain repertoire from a mammalian species or an immunized non-human mammal, the method comprising: (a) providing a plurality of Acceptor Framework nucleic acid sequences encoding distinct human immunoglobulin variable domains, each Acceptor Framework nucleic acid sequence comprising a first framework region, a second framework region, a third framework region, and a fourth framework region, wherein the FR1 and FR2 regions are interspaced by a complementarity determining region 1, the FR2 and FR3 regions are interspaced by a complementarity determining region 2, and the FR3 and FR4 regions are interspaced by a stuffer nucleic acid sequence comprising at least two Type IIs restriction enzyme recognition sites interspaced by a random nucleic acid sequence encoding a polypeptide that performs the function of a variable immunoglobulin CDR3 region; (b) providing a plurality of diversified nucleic acid sequences encoding complementarity determining region 3 sequences isolated from the mammalian species immunoglobulin repertoire or the immunized non-human mammal wherein each of the plurality of diversified nucleic acid sequences comprises a Type IIs restriction enzyme recognition site at each extremity; (c) digesting each of the plurality of nucleic acid sequences encoding the CDR3 regions using a Type IIs restriction enzyme that binds to the Type IIs restriction enzyme recognition site of step (b) and digesting the stuffer nucleic acid sequence of step (a) from the Acceptor Framework using a Type IIs restriction enzyme that binds to the Type IIs restriction enzyme recognition site of step (a); and (d) ligating the digested nucleic acid sequences encoding the CDR3 regions or the amino acid sequences of step (c) into the digested Acceptor Framework of step (c) such that the FR3 and FR4 regions are interspaced by the nucleic acid sequences encoding the CDR3 region or the amino acid sequence that can fulfill the role of a CDR3 region and a complete immunoglobulin variable domain encoding sequences that do not contain the Type IIs restriction enzyme recognition sites of steps (a) and (b) are restored. (a) providing a plurality of Acceptor Framework nucleic acid sequences encoding distinct human immunoglobulin variable domains, each Acceptor Framework nucleic acid sequence comprising a first framework region, a second framework region, a third framework region, and a fourth framework region, wherein the FR1 and FR2 regions are interspaced by a complementarity determining region 1, the FR2 and FR3 regions are interspaced by a complementarity determining region 2, and the FR3 and FR4 regions are interspaced by a stuffer nucleic acid sequence comprising at least two Type IIs restriction enzyme recognition sites interspaced by a random nucleic acid sequence encoding a polypeptide that performs the function of a variable immunoglobulin CDR3 region; (b) providing a plurality of diversified nucleic acid sequences encoding complementarity determining region 3 sequences isolated from the mammalian species immunoglobulin repertoire or the immunized non-human mammal wherein each of the plurality of diversified nucleic acid sequences comprises a Type IIs restriction enzyme recognition site at each extremity; (c) digesting each of the plurality of nucleic acid sequences encoding the CDR3 regions using a Type IIs restriction enzyme that binds to the Type IIs restriction enzyme recognition site of step (b) and digesting the stuffer nucleic acid sequence of step (a) from the Acceptor Framework using a Type IIs restriction enzyme that binds to the Type IIs restriction enzyme recognition site of step (a); and (d) ligating the digested nucleic acid sequences encoding the CDR3 regions or the amino acid sequences of step (c) into the digested Acceptor Framework of step (c) such that the FR3 and FR4 regions are interspaced by the nucleic acid sequences encoding the CDR3 region or the amino acid sequence that can fulfill the role of a CDR3 region and a complete immunoglobulin variable domain encoding sequences that do not contain the Type IIs restriction enzyme recognition sites of steps (a) and (b) are restored.