PDCD1 and neoplasm: A large number of PD-1 molecules on the surface of T cells inhibit the proliferation and activation of CD4+T cells and CD8+T cells, and some activated cytokines (such as IFN-γ) are produced to break the homeostasis of immune response, which facilitates tumor cells to evade the surveillance of the body's immune system and promotes tumor progression (Butte et al. 2008; Zhu et al. 2018).