Illumina HiSeq 3000 sequencing
gDNA from fixed cells was isolated with DNeasy Blood & Tissue Kits (#Qiagen 69504) and 4h digestion with Proteinase K. Subsequently, the sgRNA sequences were amplified for sequencing as described previously https://doi.org/10.1126/science.1247005 with GoTaq G2 Hot Start polymerase .  The PCR2 size distribution was assessed using the Bioanalyzer-2100 High Sensitivity DNA Kit (Agilent), pooled equimolar and gel-purified for the ~380 bp band (QIAquick Gel Extraction Kit, Qiagen, #28706) and quantified with the Qubit Fluorometric Quantitation system (Life Technologies).