Genome editing using CRISPR/Cas9 "gene scissors" is a powerful tool for biological discovery and for identifying novel drug targets. In pooled CRISPR screens, a large number of cells are edited simultaneously using CRISPR guide-RNAs against thousands of different genes. Next, some of the edited cells are experimentally selected, and their guide-RNAs are counted to determine which genes are most important for the studied biological mechanism. This screening method is most useful for addressing questions that are directly linked to a cell's ability to grow, for example identifying genes that protect cancer cells against chemotherapy or immune cells against HIV infection. In contrast, pooled screens