A method of obtaining an enriched population of target polynucleotides comprising: (i) providing a pool of single stranded starting oligonucleotides for the preparation of a pool of synthetic single guide RNAs (sgRNAs) for an sgRNA-guided nucleic acid-binding protein, wherein said starting oligonucleotides comprise, in 5' to 3' order, a promoter segment, a random segment and a binding segment, wherein the binding segment is complementary to at least a portion of a scaffold sequence for interaction with the sgRNA-guided nucleic acid-binding protein; (ii) providing one or more sense and/or antisense strand catcher oligonucleotides, which are complementary to at least a portion of the random segment if this random sequence comprises a target specific sequence, comprising a tag capable of binding to a cognate interactor; (iii) hybridizing said pool of starting oligonucleotides and said sense and/or antisense strand catcher oligonucleotide(s); (iv) removing complexes of starting oligonucleotides and sense strand catcher oligonucleotides and/or complexes of starting oligonucleotides and antisense strand catcher oligonucleotides from said pool of starting oligonucleotides by binding said tag to a cognate interactor, preferably located on a bead or a suitable surface, thereby obtaining a reduced pool of starting oligonucleotides; (v) preparing a pool of sgRNAs with said reduced pool of starting oligonucleotides obtained in step (iv) by hybridizing oligonucleotides with a scaffold functionality to the oligonucleotides of the reduced pool of starting oligonucleotides; performing DNA extension leading to double stranded DNA molecules comprising a promoter segment, a complementary segment and said sgRNA scaffold functionality; followed by a transcription reaction from said promoter, yielding a pool of sgRNAs; (vi) cleaving non-target polynucleotides from a mixture of polynucleotides obtained from a test sample with an sgRNA-guided nucleic acid-binding protein using the pool of sgRNAs obtained in step (v), wherein the target polynucleotides are not cleaved; and (vii) size selecting uncut target polynucleotides from said mixture of polynucleotides obtained in step (vi), thereby enriching said target polynucleotides from the mixture of polynucleotides obtained from a test sample. (i) providing a pool of single stranded starting oligonucleotides for the preparation of a pool of synthetic single guide RNAs (sgRNAs) for an sgRNA-guided nucleic acid-binding protein, wherein said starting oligonucleotides comprise, in 5' to 3' order, a promoter segment, a random segment and a binding segment, wherein the binding segment is complementary to at least a portion of a scaffold sequence for interaction with the sgRNA-guided nucleic acid-binding protein; (ii) providing one or more sense and/or antisense strand catcher oligonucleotides, which are complementary to at least a portion of the random segment if this random sequence comprises a target specific sequence, comprising a tag capable of binding to a cognate interactor; (iii) hybridizing said pool of starting oligonucleotides and said sense and/or antisense strand catcher oligonucleotide(s); (iv) removing complexes of starting oligonucleotides and sense strand catcher oligonucleotides and/or complexes of starting oligonucleotides and antisense strand catcher oligonucleotides from said pool of starting oligonucleotides by binding said tag to a cognate interactor, preferably located on a bead or a suitable surface, thereby obtaining a reduced pool of starting oligonucleotides; (v) preparing a pool of sgRNAs with said reduced pool of starting oligonucleotides obtained in step (iv) by hybridizing oligonucleotides with a scaffold functionality to the oligonucleotides of the reduced pool of starting oligonucleotides; performing DNA extension leading to double stranded DNA molecules comprising a promoter segment, a complementary segment and said sgRNA scaffold functionality; followed by a transcription reaction from said promoter, yielding a pool of sgRNAs; (vi) cleaving non-target polynucleotides from a mixture of polynucleotides obtained from a test sample with an sgRNA-guided nucleic acid-binding protein using the pool of sgRNAs obtained in step (v), wherein the target polynucleotides are not cleaved; and (vii) size selecting uncut target polynucleotides from said mixture of polynucleotides obtained in step (vi), thereby enriching said target polynucleotides from the mixture of polynucleotides obtained from a test sample.