CRISPR-Cas System: The Powerful Modulator of Accessory Genomes in Prokaryotes.Being frequently exposed to foreign nucleic acids, bacteria and archaea have developed an ingenious adaptive defense system, called CRISPR-Cas.  Different types and families of CRISPR-Cas systems consist of distinct adaptation and effector modules with evolutionary trajectories, partially independent.  A review of the most recent data regarding the structure, ecology, and evolution of CRISPR-Cas systems and their role in the modulation of accessory genomes in prokaryotes is proposed in this article.  The CRISPR-Cas system&apos;s impact on the physiology and ecology of prokaryotes, modulation of horizontal gene transfer events, is also discussed here. 
CRISPR-Cas System
The system is composed of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) array, together with CRISPR (cas)-associated genes. 
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)
The inserted segments (spacers) are transcribed and then used by cas proteins as guide RNAs for recognition and inactivation of the targets. 
cas proteins
The inserted segments (spacers) are transcribed and then used by cas proteins as guide RNAs for recognition and inactivation of the targets. 
guide RNAs
The origin of the effector modules and the mechanism of spacer integration/deletion is far less clear. 
spacer integration/deletion