A method for editing at least one endogenous chromosomal sequence in a cell by inserting a sequence into the chromosomal sequence, the method comprising: a) introducing into the cell (i) at least one targeting endonuclease or nucleic acid encoding a targeting endonuclease, the targeting endonuclease being able to introduce a double-stranded break at a targeted cleavage site in the chromosomal sequence, (ii) at least one first single-stranded nucleic acid comprising a first portion having substantial sequence identity to the chromosomal sequence on the upstream side of the targeted cleavage site, (iii) at least one second single-stranded nucleic acid comprising a first portion having substantial sequence identity to the downstream side of the targeted cleavage site, and (iv) at least one donor polynucleotide comprising a first sequence having substantial sequence identity to a second portion of the first single-stranded nucleic acid and a second sequence having substantial sequence identity to a second portion of the second single-stranded nucleic acid, and wherein the first and second sequences of the donor polynucleotide flank a sequence for insertion into the chromosomal sequence; and b) maintaining the cell under conditions such that the double stranded break introduced by the targeting endonuclease is repaired by a homology-directed process such that the sequence for insertion is integrated into the chromosomal sequence, thereby editing the chromosomal sequence; wherein the method does not comprise a process for modifying the germ line genetic identity of a human being and, wherein the method does not comprise a method for treatment of the human or animal body by surgery or therapy. a) introducing into the cell (i) at least one targeting endonuclease or nucleic acid encoding a targeting endonuclease, the targeting endonuclease being able to introduce a double-stranded break at a targeted cleavage site in the chromosomal sequence, (ii) at least one first single-stranded nucleic acid comprising a first portion having substantial sequence identity to the chromosomal sequence on the upstream side of the targeted cleavage site, (iii) at least one second single-stranded nucleic acid comprising a first portion having substantial sequence identity to the downstream side of the targeted cleavage site, and (iv) at least one donor polynucleotide comprising a first sequence having substantial sequence identity to a second portion of the first single-stranded nucleic acid and a second sequence having substantial sequence identity to a second portion of the second single-stranded nucleic acid, and wherein the first and second sequences of the donor polynucleotide flank a sequence for insertion into the chromosomal sequence; and (i) at least one targeting endonuclease or nucleic acid encoding a targeting endonuclease, the targeting endonuclease being able to introduce a double-stranded break at a targeted cleavage site in the chromosomal sequence, (ii) at least one first single-stranded nucleic acid comprising a first portion having substantial sequence identity to the chromosomal sequence on the upstream side of the targeted cleavage site, (iii) at least one second single-stranded nucleic acid comprising a first portion having substantial sequence identity to the downstream side of the targeted cleavage site, and (iv) at least one donor polynucleotide comprising a first sequence having substantial sequence identity to a second portion of the first single-stranded nucleic acid and a second sequence having substantial sequence identity to a second portion of the second single-stranded nucleic acid, and wherein the first and second sequences of the donor polynucleotide flank a sequence for insertion into the chromosomal sequence; and b) maintaining the cell under conditions such that the double stranded break introduced by the targeting endonuclease is repaired by a homology-directed process such that the sequence for insertion is integrated into the chromosomal sequence, thereby editing the chromosomal sequence; wherein