Research has demonstrated that ISGs released by cancer cells into the TME inhibit the anti-tumour function of cytotoxic T-lymphocytes (CTLs) through the activation of metabolic enzymes such as IDO1, the activation of the inhibitory receptor, programmed cell death protein 1 (PD-1) by its major ligand, programmed cell death ligand 1 (PD-L1), and extrinsic suppression by Forkhead box P3 (FoxP3+) and regulatory T cells (Tregs) [60,61]. The gene discussed is PDCD1; the disease is cancer.