CRISPR/Cas9 tools have great capacity for the diagnosis and treatment of cancer, including (1) the use of CRISPR/Cas9-based diagnostic systems SHERLOCK and DETECTR for cancer diagnostics, (2) providing TCR knockout (KO) CAR-T cells (universal CAR-T cells), (3) KO of inhibitory receptors such as PD-1 and LAG-3 to promote the capability of cancer immunotherapy, (4) elimination of oncogenic virus-like HPV, (5) and establishment of in vivo tumor models by eliciting mutations in several genes [7, 45, 95–97] (Fig. 2). The gene discussed is LAG3; the disease is cancer.