PDCD1 and neoplasm: Among the dominant challenges for the laboratory diagnosis of PD-1 and its cognate PD-L1 counterpart in tumor tissue samples are the highly variable expression dynamics within different areas of the same tissue, the lack of reliable antibodies and diagnostic kits for some types of solid tumors, the expression heterogeneity of both biomarkers in the context of TILs and tumor cells, and above all, insufficient evidence of a correlation between PD-1 or PD-L1 expression and the treatment response.