PDCD1 and neoplasm: The discovery that the PD-1/PD-L1 axis is involved in T cell regulation led to preclinical studies demonstrating that PD-1/PD-L1 is often overexpressed in malignant cells, and several in vivo experiments showed that inhibition of PD-1 caused tumor rejection in mice, whereas induced overexpression of PD-L1 led to tumor colonization and disease progression [15].