What is a CRISPR-Cas system ?

CRISPR are repeat arrays found in the DNA of many bacteria and archaea. The name is an acronym for Clustered Regularly Interspaced Short Palindromic Repeats.
The repeats or DR, ranging in size from 23 to 47 base pairs, are separated by spacers of similar length. Repeats often show some dyad symmetry but are not truly palindromic. Spacers are usually unique in a genome. They match sequences in genomes of phage, plasmid or mobile genetic elements. Inside a species, the CRISPR repeat array may show polymorphism.

Cas genes stand for CRISPR-associated genes. Together with the CRISPR array they constitute the CRISPR-Cas defense mechanism. Cas function as clusters of 3 to more than 10 genes and can be distributed into 6 types (I to VI) and more than 30 subtypes.