library_construction_protocol: Cells were harvested for sorting 14 d after transduction of the gRNA sub-library. Cells were washed once with 1x PBS, dissociated using Accutase, filtered through a 30 μm CellTrics filter (Sysmex, 04-004-2326) and resuspended in FACS Buffer (0.5% BSA (Sigma, A7906), 2 mM EDTA (Sigma, E7889) in PBS). The highest and lowest 10% of cells were sorted based on GFP expression and 0.4 x 10^6 cells were sorted into each bin. Sorting was done with a SH800 FACS Cell Sorter (Sony Biotechnology). After sorting, genomic DNA was harvested with the DNeasy Blood and Tissue Kit (Qiagen, 69506). The gRNA libraries were amplified from each gDNA sample across 100 μL PCR reactions using Q5 hot start polymerase (NEB, M0493) with 1 μg of gDNA per reaction. The PCR amplification was done according to the manufacturer's instructions, using 25 cycles at an annealing temperature of 60 °C with the following primers 18: Fwd: 5′-AATGATACGGCGACCACCGAGATCTACACAATTTCTTGGGTAGTTTGCAGTT Rev: 5′-CAAGCAGAAGACGGCATACGAGAT-(6-bp index sequence)- GACTCGGTGCCACTTTTTCAA The amplified libraries were purified with Agencourt AMPure XP beads (Beckman Coulter, A63881) using double size selection of 0.65× and then 1× the original volume. Each sample was quantified after purification with the Qubit dsDNA High Sensitivity assay kit (Thermo Fisher, Q32854). Samples were pooled and sequenced on a MiSeq (Illumina) with 21-bp paired-end sequencing using the following custom read and index primers: Read1: 5′-GATTTCTTGGCTTTATATATCTTGTGGAAAGGACGAAACACCG Index: 5′-GCTAGTCCGTTATCAACTTGAAAAAGTGGCACCGAGTC Read2: 5' - GTTGATAACGGACTAGCCTTATTTAAACTTGCTATGCTGTTTCCAGCATAGCTCTTAAAC