The present invention relates to a method of obtaining an enriched population of a target polynucleotide using a synthetic single guide RNA (sgRNA) for an sgRNA-guided nucleic acid-binding protein, as well as to a method of obtaining a pool of target-irrelevant synthetic single guide RNAs (sgRNAs) for a sgRNA-guided nucleic acid-binding protein. Also provided is a target polynucleotide and sgRNAs obtainable by the methods of the invention. Further envisaged is a kit comprising a pool of sgRNAs obtainable by the method of the invention, and the use of a pool of sgRNAs obtainable by the methods of the invention.