PDCD1 and cancer: For example, cancer cell-derived exosomal circUHRF1 exerted tumor immunosuppression by inducing NK cell exhaustion and reducing sensitivity to programmed cell death protein 1 (PD-1) blockade immunotherapy in hepatocellular carcinoma (HCC) (91); increased circFAT1 coordinately controlled cancer stemness and immune evasion through promoting signal transducer and activator of transcription 3 (STAT3) activation and inhibiting CD8+ T cell infiltration into the tumor microenvironment (92).