CRISPR-Cas encodes an adaptive immune system that defends prokaryotes against 
infectious viruses and plasmids. Immunity is mediated by Cas nucleases, which 
use small RNA guides (the crRNAs) to specify a cleavage site within the genome 
of invading nucleic acids. In type II CRISPR-Cas systems, the DNA-cleaving 
activity is performed by a single enzyme Cas9 guided by an RNA duplex. Using 
synthetic single RNA guides, Cas9 can be reprogrammed to create specific 
double-stranded DNA breaks in the genomes of a variety of organisms, ranging 
from human cells to bacteria, and thus constitutes a powerful tool for genetic 
engineering. Here we describe recent advancements in our understanding of type 
II CRISPR-Cas immunity and how these studies led to revolutionary genome editing 
applications.