Genomic DNA was extracted from samples at the beginning and end of the screen using the JetFlex Genomic DNA Purification Kit (Thermo Fisher Scientific). The integrated sgRNA cassettes were amplified by PCR (NEBNext High-Fidelity 2x PCR Master Mix, New England Biolabs), gel-purified (GeneJET Gel Extraction Kit, Thermo Fisher Scientific), and sequenced on an Illumina HiSeq 2000 instrument (50 bp single-end).