A method of sample preparation for identifying DNA double-strand breaks (DSBs) in a nucleic acid sample, wherein the preparation comprises modifying DSB-associated nucleic acids to be: (i) suitable for binding to a substrate comprising a first immobilised primer, and (ii) suitable for amplification, wherein the amplification comprises the use of said first immobilised primer and a second primer; the method comprising: a) providing a sample comprising a plurality of nucleic acids; b) exposing the plurality of nucleic acids to a first adaptor under conditions conducive to ligation, wherein the first adaptor comprises an oligonucleotide capable of being ligated to a 3' terminus of a strand of a DSB and which comprises a sequence that is capable of binding to the first immobilised primer by hybridisation; c) fragmenting the plurality of nucleic acids; and d) exposing the plurality of nucleic acids to a second adaptor under conditions conducive to ligation, wherein the second adaptor comprises an oligonucleotide capable of being ligated to a 5' terminus of a strand at a break induced by fragmentation, but is not capable of being ligated to the first adaptor, and which does not comprise a sequence that is capable of binding to a primer immobilised to the substrate by hybridisation, and which comprises a sequence identical to a region of the second primer. (i) suitable for binding to a substrate comprising a first immobilised primer, and (ii) suitable for amplification, wherein the amplification comprises the use of said first immobilised primer and a second primer; the method comprising: a) providing a sample comprising a plurality of nucleic acids; b) exposing the plurality of nucleic acids to a first adaptor under conditions conducive to ligation, wherein the first adaptor comprises an oligonucleotide capable of being ligated to a 3' terminus of a strand of a DSB and which comprises a sequence that is capable of binding to the first immobilised primer by hybridisation; c) fragmenting the plurality of nucleic acids; and d) exposing the plurality of nucleic acids to a second adaptor under conditions conducive to ligation, wherein the second adaptor comprises an oligonucleotide capable of being ligated to a 5' terminus of a strand at a break induced by fragmentation, but is not capable of being ligated to the first adaptor, and which does not comprise a sequence that is capable of binding to a primer immobilised to the substrate by hybridisation, and which comprises a sequence identical to a region of the second primer. a) providing a sample comprising a plurality of nucleic acids; b) exposing the plurality of nucleic acids to a first adaptor under conditions conducive to ligation, wherein the first adaptor comprises an oligonucleotide capable of being ligated to a 3' terminus of a strand of a DSB and which comprises a sequence that is capable of binding to the first immobilised primer by hybridisation; c) fragmenting the plurality of nucleic acids; and d) exposing the plurality of nucleic acids to a second adaptor under conditions conducive to ligation, wherein the second adaptor comprises an oligonucleotide capable of being ligated to a 5' terminus of a strand at a break induced by fragmentation, but is not capable of being ligated to the first adaptor, and which does not comprise a sequence that is capable of binding to a primer immobilised to the substrate by hybridisation, and which comprises a sequence identical to a region of the second primer.