Genomic DNA was extracted using PureLink Genomic DNA kit (ThermoFisher Scientific, K182001) Two PCR reactions were used to prepare samples for Illumina Next-Gen sequencing. The first PCR was used to amplify sgRNA inserts in the cells, and the second PCR was used to add Illumina sequencing tags as well as index tags for sample identification. To improve the complexity of the library required for deep sequencing, a mixture of 5 forward primers with staggered nucleotides immediately upstream of the sgRNA sequences was used in the second PCR. Survival screen after genome-wide KO of genes that may be important for survival under HU treatment, but not needed when Exo1 is also depleted