text: The clustered regularly interspaced short palindrome repeats (CRISPR)/Cas9 system is a gene-editing technology that can introduce double-strand breaks (DSBs) at a target genomic locus. By using a single guide RNA (sgRNA), the endonuclease Cas9 can be delivered to a specific DNA sequence where it cleaves the nucleotide chain. The specificity of the sgRNA is determined by a 20-nt sequence, homologous to the genomic locus of interest, and the binding to Cas9 is mediated by a constant scaffold region of the sgRNA. The desired target site must be immediately followed (5’ to 3’) by a conserved 3 nucleotide protospacer adjacent motif (PAM). In order to repair the DSBs, the cell may use the highly error prone non-homologous end joining, or homologous recombination. By designing suitable sgRNAs, planned insertions or deletions can be introduced into the genome. In the context of genome-wide LOF screens, the aim is to cause gene disruption and knockout.