DNA may be inserted, replaced, removed, or modified at particular locations in a genetic sequence for therapeutic benefit in order to treat cancer, rare inherited disorders, HIV, or other diseases. Several approaches rely on the use of molecular scissors, often an engineered enzyme, to make precise cuts at a specific location in the genome. The gap that results is then repaired, using healthy genetic material, to create a corrected gene.

Genome editing enzymes that are currently used in genome editing include:

• Nucleases such as Cas9 and Cas 12a that derive from Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas)
• Meganucleases
• Zinc finger nucleases (ZFNs)
• Transcription activator-like effector-based nucleases (TALEN)

Alternatively, genome editing can also be performed by homologous recombination of adeno-associated virus (AAV)-derived sequences into the patient’s DNA. 

Homologous recombination is a type of genetic recombination that occurs during meiosis (the formation of egg and sperm cells). Paired chromosomes from the male and female parent align so that similar DNA sequences from the paired chromosomes cross over each other. Crossing over results in a shuffling of genetic material and is one reason for the genetic variation and yet similarities we see in children.