The site-specific modification of an existing gene is known as gene editing. A section of DNA must be cut with an endonuclease (such as the CRISPR-Cas9 system) before the two severed ends are brought together, frequently with a new or improved sequence inserted between them. Somatic cell gene editing can be helpful in a variety of clinical contexts, and some preliminary preclinical and clinical trials have been carried out. Extremely high levels of precision are required for DNA recognition, excision, and repair; issues with publishing integrity must be resolved. Germline editing utilizing eggs, sperm, or embryos raises ethical concerns.