Question: What is the main biological function of the CRISPR-CAS9 genome editing system?

Context:
Mutagenesis and homologous recombination in Drosophila cell lines using CRISPR/Cas9.
We have applied the CRISPR/Cas9 system to Drosophila S2 cells to generate targeted genetic mutations in more than 85% of alleles. By targeting a constitutive exon of the AGO1 gene, we demonstrate homozygous mutation in up to 82% of cells, thereby allowing the study of genetic knockouts in a Drosophila cell line for the first time. We have shown that homologous gene targeting is possible at 1-4% efficiency using this system, allowing for the construction of defined insertions and deletions.
This technology enables controlled genetic manipulation in Drosophila cell lines, and its simplicity offers the opportunity to study cellular phenotypes genome-wide.
The prokaryotic CRISPR/Cas9 type II genome editing system has recently been applied in cell lines and vertebrates.
We now demonstrate that CRISPR/Cas9 mutagenesis in zebrafish is highly efficient, reaching up to 86.0%, and is heritable. The efficiency of the CRISPR/Cas9 system further facilitated the targeted knock-in of a protein tag provided by a donor oligonucleotide with knock-in efficiencies of 3.5-15.6%. 
The ease and efficiency of the CRISPR/Cas9 system with limited off-target effects make it a powerful genome engineering tool for in vivo studies.
Cas9/CRISPR has been reported to efficiently induce targeted gene disruption and homologous recombination in both prokaryotic and eukaryotic cells.
Sequence-specific nucleases like TALENs and the CRISPR/Cas9 system have greatly expanded the genome editing possibilities in model organisms such as zebrafish. Both systems have recently been used to create knock-out alleles with great efficiency, and TALENs have also been successfully employed in knock-in of DNA cassettes at defined loci via homologous recombination (HR). Here we report CRISPR/Cas9-mediated knock-in of DNA cassettes into the zebrafish genome at a very high rate by homology-independent double-strand break (DSB) repair pathways.
The type II CRISPR/Cas9 system (clustered regularly interspaced short palindromic repeats/CRISPR-associated) has recently emerged as an efficient and simple tool for site-specific engineering of eukaryotic genomes.
The CRISPR/Cas9 system has attracted significant attention for its potential to transform genome engineering. We and others have recently shown that the RNA-guided Cas9 nuclease can be employed to engineer the Drosophila genome, and that these modifications are efficiently transmitted through the germline.
Here we highlight the variety of genome modifications facilitated by the CRISPR/Cas9 system along with key considerations for starting your own CRISPR genome engineering project.
Here, we present a simple and highly efficient method for generating and detecting mutations of any gene in Drosophila melanogaster through the use of the CRISPR/Cas9 system (clustered regularly interspaced palindromic repeats/CRISPR-associated).
Recently the type II prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas) system has been adapted to serve as a targeted genome mutagenesis tool.
This CRISPR/Cas9 system represents a highly effective and scalable gene knockout method in zebrafish and has the potential for applications in other model organisms.
The CRISPR/Cas system has been adapted as an efficient gene-targeting technology with the potential for multiplexed genome editing.

Answer: The CRISPR/Cas9 system (clustered regularly interspaced short palindromic repeats/CRISPR-associated) has recently emerged as an efficient and simple tool for site-specific engineering of eukaryotic genomes. The CRISPR/Cas9 system has attracted significant attention for its potential to transform genome engineering. It has been shown that the RNA-guided Cas9 nuclease can be employed to engineer the Drosophila genome, and that these modifications are efficiently transmitted through the germline. The CRISPR/Cas9 system has been reported to efficiently induce targeted gene disruption and homologous recombination in both prokaryotic and eukaryotic cells. The CRISPR/Cas9 system has been used to create knock-out alleles with great efficiency, and it has also been employed in knock-in of DNA cassettes at defined loci via homologous recombination (HR). The ease and efficiency of the CRISPR/Cas9 system with limited off-target effects make it a powerful genome engineering tool for in vivo studies.