PDCD1 and neoplasm: Immune checkpoints contribute to the malignant progression of tumor by regulating tumor immune microenvironment and promote malignant behaviors such as tumor cell proliferation and metastasis [12], including programmed cell death 1 (PD-1) [13–15], poliovirus receptor (PVR) [16, 17], SIRPA [18, 19] and other immune checkpoints.