In future studies, many aspects remain to be clarified, including the following: (1) whether cytokines stimulate the upregulation of PD-1 or PD-L1 in an indirect manner, for example, upregulating the production of IFN; (2) if the same signal pathway components have different effects in different cancer species; (3) whether a large number of in vitro experiments are reproducible in vivo; and, (4) whether the subtypes of PD-1+ or PD-L1+ immune cells have their own role, and which effect is dominant. The gene discussed is PDCD1; the disease is cancer.