English: Cellular Repair Mechanisms of Double-Strand DNA Breaks can be exploited for Genetic Manipultions. When a double-strand break (DSB) is introduced in a cell’s DNA, there are two major repair pathways. In one pathway the blunt DNA ends are joined in a process called non-homologous end joining (NHEJ). This mechanism is error-prone and may produce insertions or deletions of bases. These alterations are collectively called Indel mutations. Indel mutations may result in a loss of function of the affected gene, e.g., by introducing a frameshift and/ or a premature stop codon. Another pathway is called homology directed repair (HDR). This pathway uses a DNA template to repair the site where the DSB occurred. When exogenous DNA is added, the sequence may serve as a template and is then integrated into the repair process. This may lead to the introduction of a new genetic sequence. Both repair pathways may result in a genetic alteration.