Lentiviral vectors have certain general limitations. For one, it is impossible to control where the viral genome integrates into the host genome, and this may affect important functions of the cell. Vannucci et al. provide an excellent review of viral vectors along with their general advantages and disadvantages. In the specific context of genome-wide CRISPR screens, producing and transducing the lentiviral particles is relatively laborious and time-consuming, taking about two weeks in total. Additionally, because the DNA integrates into the host genome, lentiviral delivery leads to long-term expression of Cas9, potentially leading to off-target effects.