https://www.scientificamerican.com/article/new-gene-editing-tool-could-fix-genetic-defects-with-fewer-unwanted-effects1/

The gene editing method CRISPR has transformed biology, giving scientists the ability to modify genes to treat or prevent genetic diseases by correcting dangerous mutations, and to create a host of new genetically modified plants and animals. But the technique, which involves using an enzyme called a nuclease that acts as molecular scissors to “cut” DNA, can cause unintended effects. Making such double-stranded breaks in DNA can result in unwanted genetic material being inserted or deleted, which can have consequences including activating genes that cause cancer. Most mutations cannot be corrected easily without creating these undesirable genetic byproducts.

In 2016, a team led by David Liu at the Broad Institute of Harvard and MIT developed another method, called base editing, which allows scientists to make precise edits to single DNA letters without relying on double-stranded breaks. This technique, however, can only be used to fix four out of the 12 types of “point” genetic mutations, which include insertions, deletions and combinations of the two.