A method for generating a covalently closed circularized (ccc) DNA based small RNA/DNA expression vector or vector library, the method comprising the steps of (a) Providing a single stranded (ss) phagemid vector comprising (i) at least one small RNA/DNA expression cassette comprising a RNA/DNA promoter and an empty target-small-RNA/DNA-sequence-introduction-site or a small RNA/DNA coding sequence and/or a DNA/RNA nuclease target sequence, or partial sequence thereof, (ii) at least one origin for replication (ORI) of single strand DNA such as a phage ORI, and in particular a f1-origin, and (b) Providing at least one species of mutagenic RNA or DNA-Primer, wherein the mutagenic RNA or DNA-primer has the following structure in 3' to 5' direction: a first homology region, a target sequence region encoding for a small RNA/DNA to be expressed, and a second homology region, wherein the first homology region is complementary to, or is capable of annealing to, a sequence of the ss-phagemid vector construct flanking the empty target-small-RNA/DNA-sequence-introduction-site or the small RNA/DNA coding sequence, or partial sequence thereof, on the 5' side, and wherein the second homology region is complementary to, or is capable of annealing to, a sequence of the ss-phagemid vector construct flanking the empty target-small-RNA/DNA-sequence-introduction-site or the small RNA/DNA coding sequence, or partial sequence thereof, on the 3' side, (c) Annealing of at least one species of mutagenic RNA or DNA-primer to the ss-phagemid vector construct and amplifying a covalently closed circularized (ccc)-heteroduplex dsDNA therefrom, (d) removing residual wild type phagemid vector DNA. (a) Providing a single stranded (ss) phagemid vector comprising (i) at least one small RNA/DNA expression cassette comprising a RNA/DNA promoter and an empty target-small-RNA/DNA-sequence-introduction-site or a small RNA/DNA coding sequence and/or a DNA/RNA nuclease target sequence, or partial sequence thereof, (ii) at least one origin for replication (ORI) of single strand DNA such as a phage ORI, and in particular a f1-origin, and (b) Providing at least one species of mutagenic RNA or DNA-Primer, wherein the mutagenic RNA or DNA-primer has the following structure in 3' to 5' direction: a first homology region, a target sequence region encoding for a small RNA/DNA to be expressed, and a second homology region, wherein the first homology region is complementary to, or is capable of annealing to, a sequence of the ss-phagemid vector construct flanking the empty target-small-RNA/DNA-sequence-introduction-site or the small RNA/DNA coding sequence, or partial sequence thereof, on the 5' side, and wherein the second homology region is complementary to, or is capable of annealing to, a sequence of the ss-phagemid vector construct flanking the empty target-small-RNA/DNA-sequence-introduction-site or the small RNA/DNA coding sequence, or partial sequence thereof, on the 3' side, (c) Annealing of at least one species of mutagenic RNA or DNA-primer to the ss-phagemid vector construct and amplifying a covalently closed circularized (ccc)-heteroduplex dsDNA therefrom, (d) removing residual wild type phagemid vector DNA. <EOS>