What is genome editing and CRISPR/Cas9？

There are actually many statements about what genome editing and CRISPR/Cas9 is and I think NIH's definition is one of the most accurate. Based on NIH's website, genome editing (also called gene editing) is a group of technologies that give scientists the ability to change an organism's DNA, and CRISPR/Ca9 (clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9)  is a faster, cheaper, more accurate, and more efficient genome editing method. Genome editing based on CRISPR-Cas9 system has several advantages as the approach. For example, it may avoid potential problems associated with plasmid-based genome-editing methods, such as uncontrolled integration of the plasmid DNA into the host genome and unwanted immune responses caused by bacterial sequences on plasmid DNA.

CRISPR-Cas9 was adapted from a naturally occurring genome editing system in bacteria. The bacteria capture snippets of DNA from invading viruses and use them to create DNA segments known as CRISPR arrays. The CRISPR arrays allow the bacteria to "remember" the viruses (or closely related ones). If the viruses attack again, the bacteria produce RNA segments from the CRISPR arrays to target the viruses' DNA. The bacteria then use Cas9 or a similar enzyme to cut the DNA apart, which disables the virus.