Before gene editing, there was gene therapy. By 1990, scientists had begun working on introducing engineered DNA into the cells of humans. The engineered DNA was to behave like a drug, counteracting against a disease-causing faulty gene. There was no effort to change the patient’s DNA. Gene therapy was initially successful. In its first trial in 1990, scientists introduced functioning copies of a missing gene into the T cells of a girl suffering from a genetic mutation that impacted her immune system. The treatment worked, and she went on to live a healthy life. However, there were soon setbacks. Gene therapy triggered fatal hyper immune response in some patients. Clearly, removing cells from a patient, injecting the cells with DNA, and reintroducing the engineered cells to their system was not the answer. Researchers looked for a way to fix genes at the source, which paved the way for the field of gene editing.

There are a couple of prerequisites for gene editing: an enzyme that can cut the double strand break in DNA, and a guide to navigate that enzyme to the site where scientists want to make the cut. Enzymes that can cut DNA or RNA are known as nucleases.