Abnormally high expression of programmed death-ligand 1 (PD-L1) on tumor cells and antigen-presenting cells in the tumor microenvironment mediates tumor immune escape, since PD-L1 binding to its receptor, programmed death-1 (PD1) on the surface of T cells, activates downstream signaling pathways and inhibits T cell activation. PD-L1 expression in cancer tissues by immunohistochemistry (IHC) is the first predictive biomarker for checkpoint blocking immunotherapy. Immune checkpoint inhibitors of anti-PD-1/PD-L1 antibodies are able to overcome the immune suppression activity by blocking the combination of PD-L1 and PD-1 and then have recently become a hot topic in cancer immunotherapy. The PD-1/PD-L1 inhibitor’s efficacy can be predicted to a certain extent by detecting PD-L1 expression in the cancer tissues. MEDx Translational Medicine has developed a PD-L1 IHC detection kit that has been used to assay the expression level of PD-L1 protein in the tumor samples.