PDCD1 and neoplasm: In these models, PD-1 expression on both CD8+ and CD4+ T cells was higher in mice with advanced MM as compared to non-tumor bearing ones; moreover, it was found a correlation between the tumor burden and the percentages of PD-1+ T cells, which were defective for the production of pro-inflammatory cytokines (IFN-γ and IL-2) after in vitro stimulation.