B2M and cancer: To validate the mutations that drive cancer evasion from T cells8, we created a library composed of single-guide (sg) RNAs targeting these 53 candidate genes (10 sgRNAs per gene) and positive control genes (Pdcd1, Cd274, Jak1, Jak2, and B2m), named as mouse Mutagenesis from Cancer Evolution, or mMCE, library (Supplementary Fig. 2a).