A new high-complexity, genome-scale CRISPR–Cas9 guide RNA (gRNA) library is presented in Cell. The Toronto KnockOut (TKO) library includes a total of 176,500 sequences that target 17,661 protein-coding genes. The authors used the library to screen five human cell lines to identify those genes for which knockouts would result in substantial fitness defects. The TKO library enabled the identification of a greater number of fitness genes than previous RNA interference or CRISPR screens. Further investigations distinguished core from context-dependent fitness genes, giving insights into cell-type-specific biological processes. A data-driven approach was used to identify context-specific oncogenic drivers.