to alter ideally within complex genomes. A ZFN is a macromolecular complex formed by monomers in which each subunit contains a zinc domain and a FokI endonuclease domain. The FokI domains must dimerize for activities, thus narrowing target area by ensuring that two close DNA-binding events occurs. The resulting cleavage event enables most genome-editing technologies to work. After a break is created, the cell seeks to repair it. A method is NHEJ, in which the cell polishes the two ends of broken DNA and seals them back together, often producing a frame shift. An alternative method is homology-directed repairs. The cell tries to fix the damage by using a copy of the sequence as a backup. By supplying their own template, researcher can have the system to insert a desired sequence instead. The success of using ZFNs in gene therapy depends on the insertion of genes to the chromosomal target area without causing damage to the cell. Custom ZFNs offer an option in human cells for gene correction. TALENs There is a method called TALENs that targets singular nucleotides. TALENs stand for transcription activator-like effector nucleases. TALENs are made by TAL effector DNA-binding domain to a DNA cleavage domain. All these methods work by as the TALENs are arranged. TALENs are "built from arrays of 33-35 amino acid modules…by assembling those arrays…researchers can target any sequence they like". This event is referred as Repeat Variable Diresidue (RVD). The relationship between the amino acids enables researchers to engineer a specific DNA domain. The TALEN enzymes are designed to remove specific parts of the DNA strands and replace the section; which enables edits to be made. TALENs can be used to edit genomes using non-homologous end joining (NHEJ) and homology directed repair. CRISPR/Cas9 The CRISPR/Cas9 system (CRISPR – Clustered Regularly Interspaced Short Palindromic Repeats, Cas9 – CRISPR-associated protein 9) is a genome editing technology based on the bacterial antiviral CRISPR/Cas system. The bacterial system has evolved to recognize viral nucleic acid sequences and cut these sequences upon recognition, damaging infecting viruses. The gene editing technology uses a simplified version of this process, manipulating the components of the bacterial system to allow location-specific gene editing. The CRISPR/Cas9 system broadly consists of two major components – the Cas9 nuclease and a guide RNA (gRNA). The gRNA contains a Cas-binding sequence and a ~20 nucleotide spacer sequence, which is specific and complementary to the target