Provided is a novel therapeutic agent for a disease caused by a dominant gene mutation (7). A therapeutic agent of the present invention comprises a donor DNA (20) that contains a polynucleotide having the following sequences (a) to (c): (a) a normal gene (1); (b) a first reverse target sequence (2a) that is located upstream of the normal gene (1) and that is cleaved by a designer nuclease; and (c) a second reverse target sequence (2b) that is located downstream of the normal gene (1) and that is cleaved by the designer nuclease, where the reverse target sequences (2a, 2b) each mean a sequence obtained by inverting a target sequence (6) that is present in the genome and that is cleaved by the designer nuclease.