The assessment of PD-L1 immunohistochemical expression and the resulting positivity rate are influenced by various factors, including different antibody clones, scoring systems and cut-off criteria for positivity, tumor heterogeneity, sample type (primary vs. metastatic tumors, RPs, biopsies, autopsies, TMAs), inter-observer variability in interpretation, and other pre-analytical variables (fixation, etc.). The gene discussed is CD274; the disease is neoplasm.