Najm, F.J. et al. Nat. Biotechnol. https://doi.org/10.1038/nbt.4048 (2017).

Genes form complex networks that underlie development and maintenance of an organism but go awry in disease. To probe pairwise genetic interactions in human cells, Najm et al. use Cas9 proteins from two bacterial species together with their optimized single guide RNAs. This dual Cas9 system, Big Papi (paired aureus and pygogenes for interactions) ensures that no recombination occurs between the two sgRNAs and has high on-target efficiency. The researchers tested for synthetic lethal interactions between 25 genes, targeted by three sgRNAs each, in six cell lines. They also probed the network of 32 proapoptotic and antiapoptotic genes. In addition, they applied Big Papi to overexpress 38 oncogenes with SpCas9–VPR while knocking out 45 tumor suppressor genes with SaCas9, and thus to screen with orthogonal activities.