To modify the DNA in most cells, scientists program the protein Cas9 to find certain repetitions in the DNA. When it does, the enzyme can snip the strands of the DNA. The cell then goes to repair the DNA, which can sometimes introduce unintended new mutations. By changing how Cas9 gets assembled and transmitted to a cell, the researchers were able to knock out genes without adding any new ones. When the researchers tested their technique on tobacco, rice, and lettuce genes, they successfully changed the desired gene about 46 percent of the time.