A method of repairing a DNA break in one or more target genomic regions via a homology directed repair (HDR) pathway, comprising: administering to one or more cells that comprise one or more target genomic regions, a genome editing system and a compound of formula ( III-E-1 ) or (III-E-2), or a pharmaceutically acceptable salt thereof, wherein: X is O or NH; Y is O, or NH; R 3 is hydrogen; R 1 is a pyrimidine ring, which is substituted by 0, 1 or 2 substituents R 2 independently selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C(=O)NHR 6 , and NR 4 R 5 ; wherein each C 1 -C 4 -alkyl and C 1 -C 4 -haloalkyl is substituted by 0, 1, or 2 OR 6 groups, each R 6 is C 1 -C 4 alkyl, R 4 and R 5 together with the N atom to which they are attached form a heterocyclic ring comprising 0 or 1 additional O or N atom wherein said heterocyclic ring may be substituted by C 1 -C 4 alkyl; and Ring B is: wherein W is N; and Z is O or S, wherein the genome editing system interacts with a nucleic acid(s) of the target genomic regions, resulting in a DNA break, and wherein the DNA break is repaired at least in part via a HDR pathway; wherein the one or more cells are cultured cells or ex vivo cells from an organism. administering to one or more cells that comprise one or more target genomic regions, a genome editing system and a compound of formula ( III-E-1 ) or (III-E-2), or a pharmaceutically acceptable salt thereof, wherein: X is O or NH; Y is O, or NH; R 3 is hydrogen; R 1 is a pyrimidine ring, which is substituted by 0, 1 or 2 substituents R 2 independently selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C(=O)NHR 6 , and NR 4 R 5 ; wherein each C 1 -C 4 -alkyl and C 1 -C 4 -haloalkyl is substituted by 0, 1, or 2 OR 6 groups, each R 6 is C 1 -C 4 alkyl, R 4 and R 5 together with the N atom to which they are attached form a heterocyclic ring comprising 0 or 1 additional O or N atom wherein said heterocyclic ring may be substituted by C 1 -C 4 alkyl; and Ring B is: wherein W is N; and Z is O or S, wherein the genome editing system interacts with a nucleic acid(s) of the target genomic regions, resulting in a DNA break, and wherein the DNA break is repaired at least in part via a HDR pathway; wherein the one or more cells are cultured cells or ex vivo cells from an organism. X is O or NH; Y is O, or NH; R 3 is hydrogen; R 1 is a pyrimidine ring, which is substituted by 0, 1 or 2 substituents R 2 independently selected from the group consisting of C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, C(=O)NHR 6 , and NR 4 R 5 ; wherein each C 1 -C 4 -alkyl and C 1 -C 4 -haloalkyl is substituted by 0, 1, or 2 OR 6 groups, each R 6 is C 1 -C 4 alkyl, R 4 and R 5 together with the N atom to which they are attached form a heterocyclic ring comprising 0 or 1 additional O or N atom wherein said heterocyclic ring may be substituted by C 1 -C 4 alkyl; and each C 1 -C 4 -alkyl and C 1 -C 4 -haloalkyl is substituted by 0, 1, or 2 OR 6 groups, each R 6 is C 1 -C 4 alkyl, R 4 and R 5 together with the N atom to which they are attached form a heterocyclic ring comprising 0 or 1 additional O or N atom wherein said heterocyclic ring may be substituted by C 1 -C 4 alkyl; and Ring B is: wherein W is N; and Z is O or S, wherein the genome editing system interacts with a nucleic acid(s) of the target genomic regions, resulting in a DNA break, and wherein the DNA break is repaired at least in part via a HDR pathway; wherein the one or more cells are cultured cells or ex vivo cells from an organism. wherein W is N; and Z is O or S, wherein the genome editing system interacts with a nucleic acid(s) of the target genomic regions, resulting in a DNA break, and wherein the DNA break is repaired at least in part via a HDR pathway; wherein the one or more cells are cultured cells or ex vivo cells from an organism. <EOS>