PDCD1 and neoplasm: Due to their upregulated checkpoint proteins and their priming against tumor neoantigens, the subpopulation of exhausted T cells (TEX) is the target of ICI therapy and increases over the course of therapy, measured by the increase of proliferative markers Ki-67 and PD-1 expressing T cells from 50% prior to therapy up to 75% in the patients’ blood (29).