Sidik, S.M. et al. Cell 167, 1423–1435 (2016).

Sidik et al. used CRISPR–Cas9 to carry out the first genome-wide genetic screen in apicomplexans, intracellular parasites that cause devastating diseases such as malaria and toxoplasmosis. To increase targeting efficiency over transient transfection, the authors constitutively expressed Cas9 in Toxoplasma gondii, in addition to a 'decoy' single guide RNA (sgRNA) to mitigate toxic overactivity. They used this strain to transfect an sgRNA library redundantly targeting each of the more than 8,000 genes in the genome, and they followed up with fitness assays. The work uncovered approximately 200 genes that were not previously known to be essential, including one encoding a broadly conserved protein they dub claudin-like apicomplexan microneme protein (CLAMP), which is required for host cell invasion.