Costimulation of 4-1BB has the potential to complement and enhance antitumor activity of PD-1/PD-L1 blockade by improving functionality and survival of chronically stimulated CD8+ T cells in the tumor microenvironment (TME; refs. 12, 13) and expanding T-cell clonality (14) and the overall prevalence of T cells (15). The gene discussed is PDCD1; the disease is neoplasm.