Clustered regularly-interspaced short palindromic
repeats (abbreviated as CRISPR, pronounced
crisper[2]) are segments of prokaryotic DNA
containing short repetitions of base sequences.
Each repetition is followed by short segments
of "spacer DNA" from previous exposures to
a bacterial virus or plasmid.[3]
The CRISPR/Cas system is a prokaryotic immune
system that confers resistance to foreign
genetic elements such as plasmids and phages,[4][5]
and provides a form of acquired 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 bacteria genomes and 90% of sequenced
archaea.[6][note 1]
The CRISPR interference technique has enormous
potential application, including altering
the germline of humans, animals and other
organisms, and modifying the genes of food
crops.
By delivering the Cas9 protein and appropriate
guide RNAs into a cell, the organism's genome
can be cut at any desired location.[7][8][9]
CRISPRs have been used in concert with specific
endonuclease enzymes for 
genome 
editing and gene regulation in species throughout
the tree 
of 
life.[10] Ethical concerns have been expressed
about this nascent biotechnology and the prospect
of 
editing 
the human germline.
