Sequencing and read count analysis
Equal molar amounts PCR gel-purified samples were pooled, denatured and diluted to 1.8pM, of which 1.3 ml was sequenced on a NextSeq 550 using a high output 75 cycle sequencing kit (Illumina). This yielded between 35 and 62 million reads per sample. Sequencing reads were aligned to the Brunello library file and counted using MAGeCK (Chen et al., 2018; Li et al., 2014). The raw read counts table are in NCBI GEO (accession number GSE206996 ).