Clustered regularly interspaced short palindromic repeats(CRISPR, pronounced crisper^[2]) are segments of prokaryoticDNA containing short repetitions of base sequences. Each repetition is followed by short segments of “spacer DNA” from previous exposures to a bacteriophage virus or plasmid.^[3]

The CRISPR/Cas system is a prokaryotic immune system that confers resistance to foreign genetic elements such as those present within plasmids and phages,^[4][5][6] and provides a form ofacquired immunity. CRISPR spacers recognize and cut these exogenous genetic elements in a manner analogous to RNA interference in eukaryotic organisms.^[3] CRISPRs are found in approximately 40% of sequenced bacterial genomes and 90% of sequenced archaea.^{[7][note 1]}

By delivering the Cas9 nuclease and appropriate guide RNAs into a cell, the cell’s genome can be cut at a desired location, allowing existing genes to be removed and/or new ones added.^[8][9][10]CRISPRs have been used in concert with specific endonucleaseenzymes for genome editing and gene regulation in species throughout the tree of life.^[11]

Cas9 was the first nuclease discovered, followed by Cpf1, which was discovered in the CRISPR/Cpf1 system of Francisella novicida.^[12][13]Other such systems are thought to exist.^[14]

CRISPR/C2c2 from the bacterium Leptotrichia shahii is RNA-guided CRISPR system that targets RNA rather than DNA, and can either cleave single-stranded RNA targets or knock them down.^[15]

The CRISPR interference technique has many potential applications, including altering the germline of humans, animals, and food crops. The use of CRISPR for genome editing^[16][17] was the AAAS‘s choice for breakthrough of the year in 2015.^[18] Bioethical concerns have been expressed about the prospect of using this nascent biotechnology for editing the human germline.^[19]

(From: Wikipedia, June 2016)