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While most CRISPR screens to date have been conducted in cell‐based in vitro models, in vivo CRISPR screens have the advantage of directly interrogating gene functions and revealing molecular mechanisms in the native context of animals. In 2015, the first in vivo CRISPR screen to identify loss‐of‐function mutations that promote tumor growth and metastasis was published (Chen et al. 2015). In this study, the authors mutagenized a nonmetastatic mouse cancer cell line using a genome‐wide gRNA library and the resulting pooled cell library was subcutaneously transplanted into immunocompromised mice. By sequencing the enriched gRNAs in the late‐stage primary tumors and lung metastases, they discovered and subsequently validated a small set of genes whose disruptions drove tumor growth and metastasis in vivo. Similar in vivo CRISPR screening approaches using xenograft mouse models have been applied to identify tumor suppressors (Katigbak et al. 2016; Song et al. 2017; Takeda et al. 2019), oncogenes (Kodama et al. 2017), and synthetic lethal drug targets (Yau et al. 2017). CRISPR screening has also been applied in syngeneic models, which have full murine immunity and comprehensive stroma, providing a more relevant setting to assess gene functions in tumor immunity and immunotherapy response. In 2017, Manguso and colleagues discovered previously unidentified cancer immunotherapy targets by transducing a focused library into mouse melanoma tumor cells and comparing the gRNA library representation in implanted tumors from immunotherapy‐treated wild‐type animals to tumors growing in Tcra−/− mice (Manguso et al. 2017). In vivo screening approaches have also been deployed for the identification of genes affecting T cell proliferation and antitumor function (Dong et al. 2019), metabolism‐associated factors in T‐cell‐mediated antitumor immunity (Wei et al. 2019), membrane targets in CD8+ murine models of glioblastoma (Ye et al. 2019), and for the facile perturbation of innate and adaptive immune cells in vivo without ex vivo manipulation of these mature lineages (LaFleur et al. 2019).